"Getting Started" is already *very* long; this shrinks it a bit, and
puts similar info closer together.

This is advanced info that most contributors won't need to know; and for
experienced contributors, it means the info is at the top of the page
instead of needing to scroll.

There was a lot of information duplicated between the two, and it wasn't
clear which one to look. This commit changes `contributing.md` to be
strictly for contribution procedures, and moves "what should I work on"
sections to "Getting Started".

This also consolidates the links in `about-this-guide.md` rather than
spreading them between about-this-guide, getting-started, and
contributing.

I suggest using https://rustc-dev-guide.org/git.html#moving-large-sections-of-code to review this commit.