Rollup merge of #113394 - joshtriplett:style-edition-snapshot, r=calebcartwright

style-guide: Document style editions, start 2024 style edition

Link to a snapshot for the 2015/2018/2021 style edition.

This is a draft, because I'd like to wait for a few style guide fixes to merge
before snapshotting the 2015/2018/2021 style edition:

- https://github.com/rust-lang/rust/pull/113145
- https://github.com/rust-lang/rust/pull/113380
- https://github.com/rust-lang/rust/pull/113384
- https://github.com/rust-lang/rust/pull/113385
- https://github.com/rust-lang/rust/pull/113386
- https://github.com/rust-lang/rust/pull/113392

I'd like to wait for these for two reasons: to make it easier to see the
differences between the 2015/2018/2021 style edition and the 2024 style
edition (without the noise of guide-wide changes), and to minimize confusion so
that bugfixes to the style guide that we include in the previous edition don't
look like they're only part of the 2024 style edition.

I've used "Miscellaneous `rustfmt` bugfixes" as a starting point for the list
of 2024 changes, for now. We can update that when we add more 2024 changes.

The section added in this PR can then serve as a baseline for our drafts of
2024 style edition changes.

In the meantime, I'd like to get someone from `@rust-lang/style` to review and
approve the text here; I'll update it with a commit hash when the above PRs
have merged.

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