Rearrange CONTRIBUTING.md into "Contributor's Guide" devdocs chapter
This moves the various sections of CONTRIBUTING.md into their own
pages in a new "Contributor's Guide" chapter of the devdocs. I think
this organization makes more sense as CONTRIBUTING.md is a bit of a
weird mix, between widely applicable things (how to sign up to GitHub)
and very niche things (e.g. how to prepare a backport). I think this
also sets the stage for writing more specific documentation (e.g. my
new jldoctest page) that would have been far too in-depth to put directly
into CONTRIBUTING.md.