Add tooltip with required components for trait tags on docs.rs

[Weihnachtsbaum]

Objective

To see a component's required components in the docs, you first have to find the Component impl which can be annoying.

Solution

Add a tooltip to the Component and Immutable Component trait tags which lists the required components. The components in that list link to their own doc page.

Testing

Build the docs locally:

RUSTDOCFLAGS="--html-after-content docs-rs/trait-tags.html --cfg docsrs_dep" RUSTFLAGS="--cfg docsrs_dep" cargo doc --no-deps -p <some_bevy_package>

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[mockersf]

I don't think we want to go in that direction.

The plan is to upstream the tags to rustdoc, and I don't see this tooltip as having enough general value for other crates.
This is also not very discoverable

[Weihnachtsbaum]

The plan is to upstream the tags to rustdoc

I didn't know that. I guess the tooltips could still be used for the bevy docs with the custom html, even if the trait tags get upstreamed, but I see that it could complicate things.

This is also not very discoverable

If we want people to know about it, we can just mention it in the release notes. Also I think that actually a lot of users would just find it out by accidentally hovering over the tags. And even if that isn't the case, I still think that something that's useful should be kept, even if not everyone knows about it.

But of course cost of maintaining vs. value has to be considered, which I can't assess.