Documentation for ECS internals

[alice-i-cecile]

Objective

• The internals of Bevy ECS are very intimidating.
• They are also poorly documented.
• Nevertheless, they are both important to not mess up and very intriguing for a certain class of new contributor / user.
• Closes Reflection allows users to clone arbitrary reflected components, but Reflect does not have a Clone bound #3372.

Solution

• Document all the things.
• Also improve existing documentation within bevy_ecs where I see the opportunity.

Status

This is a draft PR, so y'all can complain about my explanations as I work.

Unless there are strong objections, I'd like to tackle most of the crate in one pass, so then I can ensure that things aren't missed and the concepts are explained coherently.

[DJMcNab]

With sparse set components, these can contain many sets of component, although each set is uniquely assigned to a single archetype.

[Nilirad]

Document all the things.

Start by putting #![warn(missing_docs)]

[alice-i-cecile]

Start by putting #![warn(missing_docs)]

I think you mean #![forbid(missing_docs)]. One day!

[Carter0]

Thank you for doing this! I was planning to look over this code at some point and this will be helpful

[cart]

Document all the things

I propose documenting sections of bevy_ecs in multiple prs instead of all at once to make reviewing + merging easier. I won't block on that, but it has my preference.

[alice-i-cecile]

I propose documenting sections of bevy_ecs in multiple prs instead of all at once to make reviewing + merging easier. I won't block on that, but it has my preference.

Sounds good @cart. I'm probably going to work in this PR, then close it out and cherrypick the commits into a dozen mini-PRs.

[alice-i-cecile]

Alright, I think that's about all of it for a first pass. I'm going to set this down for a couple of days, and then come back to split the PRs and close this out when it's done.

Most of these docs could do with being expanded with context and examples, but that will be better handled in small PRs that experts can quickly review.

[alice-i-cecile]

Split into #3384, #3385, #3386, #3387, #3388, #3389, #3390 and #3391 😂 Closing this out now.