[Merged by Bors] - Migration guide 0.9

[IceSentry]

Migration guide generated by #469

This is mostly just the generated output with a tiny bit of manual modifications.

[rparrett]

I added C-Breaking-Change to a few PRs that were definitely breaking and had migration guides.

edit: resolved

[rparrett]

Compiling notes here for additions or changes. Deleting these as I address them. Feel free to pick one up if I haven't done it yet.

### Whole document

Standardize on
// Old (Bevy 0.8)
// New (Bevy 0.9)

Make a second pass at overall order

### 6000
The mouse changes will be reverted before release, so this text needs updating
Needs formatting

### 6345 and 6354 and 5723
Can we create a smaller but just-as-illustrative scene example?
Can we do even better an include an enum and tuple struct?

### 5340
Code block might be more clear for novice rust users or non-english-speakers.

[IceSentry]

@rparrett you can suggest changes, as long as it isn't anything too big. When I generate the guide again I make sure to only stage the new guides and keep changes to anything already modified

[ickk]

I finished a review of every PR in this list looking for breaking changes that weren't already labelled.
https://github.com/bevyengine/bevy/pulls?q=is%3Apr+closed%3A%3E%3D2022-07-31+%22merged+by+bors%22+
Thanks @rparrett for catching a few.

I identified 4 PRs that I didn't make a decision on one way or the other:

• [Merged by Bors] - dynamic scene builder bevy#6227
  TypeRegistryArc becomes AppTypeRegistry in various places.

• [Merged by Bors] - Add reflect(skip_serializing) which retains reflection but disables automatic serialization bevy#5250
  Doesn't change or remove any existing functionality as far as I can tell, however seems like it newly disallows #[ignore] attributes on enum variants (which according to the PR didn't do anything anyway) so might cause existing code not to compile.

• [Merged by Bors] - make WorldQuery very flat bevy#5205
  Basic migration notes included, but I'm not sure how many users will be manually implementing WorldQuery.

• [Merged by Bors] - bevy_reflect: Reflect enums bevy#4761
  Breaks the serialisation format, however later commits also break the serialisation format in even greater ways. Probably would be best to consolidate advice for scene migrations to a single section.

[rparrett]

Quick skim for now.

[rparrett]

Combined the scene format changes here: IceSentry#6

[laundmo]

Just a heads up, while applying this guide, I noticed that there isn't any documentation for the reworked PluginGroup Trait usage, especially concering PluginGroupBuilder, yet.

Heres the changes which i needed:

• change the build signature from fn build(&mut self, group: &mut bevy::app::PluginGroupBuilder) to fn build(self) -> PluginGroupBuilder
• add let mut group = PluginGroupBuilder::start::<Self>(); at the start
• change group.add to group = group.add
• change #[cfg(not(feature = "myfeature"))] to use blocks like

  #[cfg(not(feature = "myfeature"))]
  {
      group = group.add(SomePlugin);
  }

[mockersf]

that looks like changes from bevyengine/bevy#6336. the migration guide for this one focused on plugin group user rather than plugin group builder

[cart]

bors r+

[cart]

Great work everyone!

[bors]

Pull request successfully merged into master.

Build succeeded:

• build-website
• generate-assets
• generate-errors
• generate-wasm-examples