Document bevy_ecs::storage

[james7132]

Objective

Move bevy_ecs towards being able to use warn(missing_docs). Aiming to partially address #3492.

Solution

Document bevy_ecs::storage as thoroughly as possible. Added module level, API level, and type level docs wherever possible, including pub(crate) APIs for easier onboarding of newer contributors.

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Changelog

Added: Documentation for all bevy::ecs::storage APIs.

[alice-i-cecile]

Very nice stuff: these are pretty small corrections and requests.

[rparrett]

Just want to +1 all of maniwani's feedback.

[james7132]

@maniwani thanks for helping wordsmith the docs here. Might need to update guidance on our general docs guidance to match.

Also, side note, maybe we should rename these to match their use as replacements for HashSet and HashMap.

* `SparseArray` -> `SparseSet`

* `SparseSet` -> `SparseMap`

I'm not 100% convinced since we very much still use array-like instead of set like terminology for SparseArray, and conceptually we still think of it as one wherever it's used. Happy to discuss this further, but not in this docs PR.