Add set_parent and remove_parent to EntityCommands (#6189)

I found myself doing
```rust
let child = commands.spawn(..).id();
commands.entity(parent).add_child(child);
```
When that could just be
```rust
commands.spawn(..).set_parent(parent);
```

Adding `set_parent` was trivial as it's just an `AddChild` command. Most of the changes are for `remove_parent`.
Also updated some outdated docs.

Co-authored-by: devil-ira <justthecooldude@gmail.com>

crates/bevy_hierarchy/src/child_builder.rs

@@ -155,7 +155,7 @@ impl Command for InsertChildren {
     }
 }
 
-/// Command that pushes children to the end of the entity's children
+/// Command that pushes children to the end of the entity's [`Children`].
 #[derive(Debug)]
 pub struct PushChildren {
     parent: Entity,
@@ -175,7 +175,7 @@ impl Command for PushChildren {
     }
 }
 
-/// Command that removes children from an entity, and removes that child's parent and inserts it into the previous parent component
+/// Command that removes children from an entity, and removes that child's parent.
 pub struct RemoveChildren {
     parent: Entity,
     children: SmallVec<[Entity; 8]>,
@@ -187,6 +187,27 @@ impl Command for RemoveChildren {
     }
 }
 
+/// Command that removes the parent of an entity, and removes that entity from the parent's [`Children`].
+pub struct RemoveParent {
+    child: Entity,
+}
+
+impl Command for RemoveParent {
+    fn write(self, world: &mut World) {
+        if let Some(parent) = world.get::<Parent>(self.child) {
+            let parent_entity = parent.get();
+            remove_from_children(world, parent_entity, self.child);
+            world.entity_mut(self.child).remove::<Parent>();
+            if let Some(mut events) = world.get_resource_mut::<Events<_>>() {
+                events.send(HierarchyEvent::ChildRemoved {
+                    child: self.child,
+                    parent: parent_entity,
+                });
+            }
+        }
+    }
+}
+
 /// Struct for building children onto an entity
 pub struct ChildBuilder<'w, 's, 'a> {
     commands: &'a mut Commands<'w, 's>,
@@ -288,6 +309,10 @@ pub trait BuildChildren {
     /// will have those children removed from its list. Removing all children from a parent causes its
     /// [`Children`] component to be removed from the entity.
     fn add_child(&mut self, child: Entity) -> &mut Self;
+    /// Sets the parent of this entity.
+    fn set_parent(&mut self, parent: Entity) -> &mut Self;
+    /// Removes the parent of this entity.
+    fn remove_parent(&mut self) -> &mut Self;
 }
 
 impl<'w, 's, 'a> BuildChildren for EntityCommands<'w, 's, 'a> {
@@ -346,6 +371,18 @@ impl<'w, 's, 'a> BuildChildren for EntityCommands<'w, 's, 'a> {
         self.commands().add(AddChild { child, parent });
         self
     }
+
+    fn set_parent(&mut self, parent: Entity) -> &mut Self {
+        let child = self.id();
+        self.commands().add(AddChild { child, parent });
+        self
+    }
+
+    fn remove_parent(&mut self) -> &mut Self {
+        let child = self.id();
+        self.commands().add(RemoveParent { child });
+        self
+    }
 }
 
 /// Struct for adding children to an entity directly through the [`World`] for use in exclusive systems