Split up contributing.md

[jyn514]

fixes #2991. see there for more context.

i highly recommend reviewing this commit-by-commit using git log --reverse -p --color-moved=dimmed-zebra --color-moved-ws=allow-indentation-change origin/main... Github's diff view is pretty useless here.

i kept "Bug reports" in contributing.md because it's linked very widely (including in rust-lang/rust/CONTRIBUTING.md) and i couldn't think of a better place for it. i don't think "Reverting a PR" really belongs in the dev-guide at all except as a link to Forge, but i kept it in contributing.md for now.

i kept the PR lifecycle even though i'm not quite satisfied with it, i think it needs to be handled in combination with improving the "Walkthrough" chapter and i'd rather not make this PR any larger than it is already.

disclosure: i used an LLM for most of this and reviewed its work. i wrote most of the last commit as well as the section titles. i used an LLM to split this into atomic commits, and edited its commit titles.

commits:

• Add a chapter for contributing to the dev-guide itself
• Move "compiler documentation" contributing docs to their dedicated chapter
• Move general contribution guidance to getting-started.md
• Move "new feature" procedures to their dedicated chapter
• Move performance guidance to tests/perf.md
• Replace moved procedures with compatibility links
• make contributing.md order make more sense

r? rustc-dev-guide

[jyn514]

in retrospect all this probably doesn't fit here. hm. i guess i can move it back to contributing.md? not sure where else it could go.

View changes since the review

[jyn514]

i wonder if (per #775) this needs to be done in combination with new walkthrough sections, rather than in isolation ... i have a draft locally i can push if it's helpful, but unfortunately it does blow up the size of the PR.

[jyn514]

ok, i shrank this PR instead of growing it, i'll work on further improvements in a separate branch so they don't make this harder to review.

[jyn514]

opened #2996 separately, and main...jyn514:rustc-dev-guide:pr-lifecycle building on both. but i tried to keep this PR small.