## Objective
The upcoming Bevy Book makes many references to the API documentation of bevy.
Most references belong to the first two chapters of the Bevy Book:
- bevyengine/bevy-website#176
- bevyengine/bevy-website#182
This PR attempts to improve the documentation of `bevy_ecs` and `bevy_app` in order to help readers of the Book who want to delve deeper into technical details.
## Solution
- Add crate and level module documentation
- Document the most important items (basically those included in the preludes), with the following style, where applicable:
- **Summary.** Short description of the item.
- **Second paragraph.** Detailed description of the item, without going too much in the implementation.
- **Code example(s).**
- **Safety or panic notes.**
## Collaboration
Any kind of collaboration is welcome, especially corrections, wording, new ideas and guidelines on where the focus should be put in.
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### Related issues
- Fixes#2246
# Objective
There is currently a 1-to-1 mapping between components and real rust types. This means that it is impossible for multiple components to be represented by the same rust type or for a component to not have a rust type at all. This means that component types can't be defined in languages other than rust like necessary for scripting or sandboxed (wasm?) plugins.
## Solution
Refactor `ComponentDescriptor` and `Bundle` to remove `TypeInfo`. `Bundle` now uses `ComponentId` instead. `ComponentDescriptor` is now always created from a rust type instead of through the `TypeInfo` indirection. A future PR may make it possible to construct a `ComponentDescriptor` from it's fields without a rust type being involved.
