Improve Query's top-level documentation (#18622)

# Objective

- There's been several changes to `Query` for this release cycle, and
`Query`'s top-level documentation has gotten slightly out-of-date.
- Alternative to #18615.

## Solution

- Edit `Query`'s docs for consistency, clarity, and correctness.
- Make sure to group `get()` and `get_many()` together instead of
`single()` and `get_many()`, to enforce the distinction from
#18615 (comment).
- Reformat doc tests so they would be readable if extracted into their
own file. (Which mainly involves adding more spacing.)
- Move link definitions to be nearer where they are used.
- Fix the tables so they are up-to-date and correctly escape square
brackets `\[ \]`.

## Testing

I ran `cargo doc -p bevy_ecs --no-deps` to view the docs and `cargo test
-p bevy_ecs --doc` to test the doc comments.

## Reviewing

The diff is difficult to read, so I don't recommend _just_ looking at
that. Instead, run `cargo doc -p bevy_ecs --no-deps` locally and read
through the new version. It should theoretically read smoother with less
super-technical jargon. :)

## Follow-up

I want to go through some of `Query`'s methods, such as `single()`,
`get()`, and `get_many()`, but I'll leave that for another PR.

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Co-authored-by: Chris Russell <[email protected]>

Author: BD103