Add basic release content tagging workflow (#18568)

# Objective

This PR begins integrating the new releate-content drafting process
(https://github.com/bevyengine/bevy/pull/18427) into our GitHub
workflows. It's similar to what we had before: Messages are posted to
PRs tagged with `M-Needs-Release-Note` or `M-Needs-Migration-Guide`
asking them to add the required material and linking to the
instructions. These messages do not trigger if the PR already has
modified files in the `release-notes` or `migration-guides` directories
(respectively).

I have also re-arranged and content slightly (to remove the need for a
directory with the current version number), tweaked the language, and
switched the templates to use the [standard markdown frontmatter
format](https://jekyllrb.com/docs/front-matter/).

## Reviewer Questions

+ Do we want to add a CI rule actually requiring tagged PRs to
create/modify files in the correct directories, or is the message prompt
enough?
+ Do we want to add a CI rule to lint the metadata, for example to
enforce that the PR number is included in the files it modifies?

.github/workflows/action-on-PR-labeled.yml

@@ -12,19 +12,63 @@ permissions:
   pull-requests: 'write'
 
 jobs:
-  comment-on-breaking-change-label:
+  comment-on-migration-guide-label:
     runs-on: ubuntu-latest
-    if: github.event.label.name == 'M-Needs-Migration-Guide' && !contains(github.event.pull_request.body, '## Migration Guide')
+    if: github.event.label.name == 'M-Needs-Migration-Guide'
     steps:
+      - uses: actions/checkout@v4
+        with:
+          ref: ${{ github.event.pull_request.head.sha }}
+          fetch-depth: 2
+      - name: Get changes
+        id: get_changes
+        shell: bash {0}
+        run: |
+          git fetch --depth=1 origin $BASE_SHA
+          git diff --exit-code $BASE_SHA $HEAD_SHA -- ./release-content/migration-guides
+          echo "found_changes=$?" >> $GITHUB_OUTPUT
+        env:
+          BASE_SHA: ${{ github.event.pull_request.base.sha }}
+          HEAD_SHA: ${{ github.event.pull_request.head.sha }}
       - uses: actions/github-script@v7
+        if: steps.get_changes.outputs.found_changes == '0'
         with:
           script: |
             await github.rest.issues.createComment({
               issue_number: context.issue.number,
               owner: context.repo.owner,
               repo: context.repo.repo,
-              body: `It looks like your PR is a breaking change, but you didn't provide a migration guide.
+              body: `It looks like your PR is a breaking change, but **you didn't provide a migration guide**.
 
-              Could you add some context on what users should update when this change get released in a new version of Bevy?
+              Please review the [instructions for writing migration guides](https://github.com/bevyengine/bevy/tree/main/release-content/migration_guides.md), then expand or revise the content in the [migration guides directory](https://github.com/bevyengine/bevy/tree/main/release-content/migration-guides) to reflect your changes.`
-              It will be used to help writing the migration guide for the version. Putting it after a \`## Migration Guide\` will help it get automatically picked up by our tooling.`
+            })
+  comment-on-release-note-label:
+    runs-on: ubuntu-latest
+    if: github.event.label.name == 'M-Needs-Release-Note'
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          ref: ${{ github.event.pull_request.head.sha }}
+          fetch-depth: 2
+      - name: Get changes
+        id: get_changes
+        shell: bash {0}
+        run: |
+          git fetch --depth=1 origin $BASE_SHA
+          git diff --exit-code $BASE_SHA $HEAD_SHA -- ./release-content/release-notes
+          echo "found_changes=$?" >> $GITHUB_OUTPUT
+        env:
+          BASE_SHA: ${{ github.event.pull_request.base.sha }}
+          HEAD_SHA: ${{ github.event.pull_request.head.sha }}
+      - uses: actions/github-script@v7
+        if: steps.get_changes.outputs.found_changes == '0'
+        with:
+          script: |
+            await github.rest.issues.createComment({
+              issue_number: context.issue.number,
+              owner: context.repo.owner,
+              repo: context.repo.repo,
+              body: `It looks like your PR has been selected for a highlight in the next release blog post, but **you didn't provide a release note**.
+
+              Please review the [instructions for writing release notes](https://github.com/bevyengine/bevy/tree/main/release-content/release_notes.md), then expand or revise the content in the [release notes directory](https://github.com/bevyengine/bevy/tree/main/release-content/release_notes) to showcase your changes.`
             })

release-content/README.md

@@ -0,0 +1,3 @@
+# Release Content
+
+This directory contains drafts of documentation for the current development cycle, which will be published to the website during the next release. You can find more information in the [release notes](./release_notes.md) and [migration guide](./migration_guide.md) files.

release-content/migration_guides.md

@@ -11,14 +11,15 @@ Bevy asks authors (and reviewers) to write a draft migration guide as part of th
 ## Where to put your migration guides
 
 Each major Bevy version (e.g. 0.12, or 2.0) will get its own migration guide.
-The draft migration guides are organized into a folder of the same name inside of `bevyengine/bevy/working-migration-guides`.
+The draft migration guides for the current cycle are organized in the `bevyengine/bevy/release-content/migration-guides` folder.
 
-When we publish our first release candidate for a cycle, these notes are moved from `bevyengine/bevy`, and into `bevyengine/bevy-website`,
+When we publish our first release candidate for a cycle, these notes are merged together and moved from `bevyengine/bevy` into `bevyengine/bevy-website`,
 where they will receive a final editing pass.
 
-## What to put in your draft migration guide
+If your PR introduces a new breaking change relative to the previous version, you should start a new guide by copying [the template](./migration_guides_template.md) into a new file in the `migration-guides` folder.
+You should also update the existing migration guides in the other files, if your change effects them.
 
-A `template.md` file is provided in `bevyengine/bevy/working-migration-guides`: copy-paste that to get started!
+## What to put in your draft migration guide
 
 Migration guides are intended to briefly communicate:
 

release-content/migration_guides_template.md

@@ -0,0 +1,30 @@
+---
+title: Feature that broke
+pull_requests: [14791, 15458, 15269]
+---
+
+Copy the contents of this file into a new file in `./migration-guides`, update the metadata, and add migration guide content here.
+
+## Goals
+
+Aim to communicate:
+
+- What has changed since the last release?
+- Why did we make this breaking change?
+- How can users migrate their existing code?
+
+## Style Guide
+
+Keep it short and sweet:
+
+- Use bullet points and make sure it's searchable.
+- Avoid headings. If you must, use only level-two headings.
+- Use backticks for types (e.g. `Vec<T>`) in either the title or the body.
+- Diff codeblocks can also be useful for succinctly communicating changes.
+
+```diff
+fn my_system(world: &mut World) {
++ world.new_method();
+- world.old_method();
+}
+```

release-content/release_notes.md

@@ -11,14 +11,14 @@ Bevy asks authors (and reviewers) to write draft release notes as part of the pu
 ## Where to put your release notes
 
 Each major Bevy version (e.g. 0.12, or 2.0) will get its own set of release notes.
-The draft release notes are organized into a folder of the same name inside of `bevyengine/bevy/working-release-notes`.
+The draft release notes are organized in the `bevyengine/bevy/release-content/release-notes` folder.
 
-When we publish our first release candidate for a cycle, these notes are moved from `bevyengine/bevy`, and into `bevyengine/bevy-website`,
+When we publish our first release candidate for a cycle, these notes are merged together and moved from `bevyengine/bevy` into `bevyengine/bevy-website`,
 where they will receive a final editing pass and any multimedia.
 
-## What to put in your draft release notes
+To start a new release note, copy-paste [the template](./release_notes_template.md) into a new file in the `release-notes` folder.
 
-A `template.md` file is provided in `bevyengine/bevy/working-release-notes`: copy-paste that to get started!
+## What to put in your draft release notes
 
 Release notes are intended to capture the essence of:
 

release-content/release_notes_template.md

@@ -0,0 +1,19 @@
+---
+title: Feature name
+authors: ["@FerrisTheCrab", "@BirdObsessed"]
+pull_requests: [14791, 15458, 15269]
+---
+
+Copy the contents of this file into `./release-notes`, update the metadata, and add release note content here.
+
+## Goals
+
+Answer the following:
+
+- What has been changed or added?
+- Why is this a big deal for users?
+- How can they use it?
+
+## Style Guide
+
+You may use markdown headings levels two and three, and must not start with a heading. Prose is appreciated, but bullet points are acceptable. Copying the introduction from your PR is often a good place to start.

working-migration-guides/template.md

@@ -1,9 +0,0 @@
-# Feature that broke (remember to update the file name)
-
-prs = [14791, 15458, 15269]
-
-Add migration guide content here. Use bullet points, make sure it's searchable. Briefly communicate:
-
-- what has changed since the last release?
-- why did we make this breaking change?
-- how can users migrate their existing code?

working-release-notes/template.md

@@ -1,11 +0,0 @@
-# Feature Name (remember to update the file name to match)
-
-authors = ["@FerrisTheCrab", "@BirdObsessed"]
-contributors = ["@HelpfulHelper"]
-prs = [14791, 15458, 15269]
-
-Add release note content here. Answer:
-
-- what has been changed or added?
-- why is this a big deal for users?
-- how can they use it?