Split the bevy_ecs reflect.rs module (bevyengine#8834)

# Objective

- Cleanup the `reflect.rs` file in `bevy_ecs`, it's very large and can
get difficult to navigate

## Solution

- Split the file into 3 modules, re-export the types in the
`reflect/mod.rs` to keep a perfectly identical API.
- Add **internal** architecture doc explaining how `ReflectComponent`
works. Note that this doc is internal only, since `component.rs` is not
exposed publicly.

### Tips to reviewers

To review this change properly, you need to compare it to the previous
version of `reflect.rs`. The diff from this PR does not help at all!
What you will need to do is compare `reflect.rs` individually with each
newly created file.

Here is how I did it:

- Adding my fork as remote `git remote add nicopap
https://github.com/nicopap/bevy.git`
- Checkout out the branch `git checkout nicopap/split_ecs_reflect`
- Checkout the old `reflect.rs` by running `git checkout HEAD~1 --
crates/bevy_ecs/src/reflect.rs`
- Compare the old with the new with `git diff --no-index
crates/bevy_ecs/src/reflect.rs crates/bevy_ecs/src/reflect/component.rs`

You could also concatenate everything into a single file and compare
against it:

- `cat
crates/bevy_ecs/src/reflect/{component,resource,map_entities,mod}.rs >
new_reflect.rs`
- `git diff --no-index  crates/bevy_ecs/src/reflect.rs new_reflect.rs`