[docs] reorganize contributors guide

[benmccann]

Moves things around a bit, so it flows a bit better. See the rendered version here: https://github.com/sveltejs/kit/blob/contrib/README.md

Some changes:

• We had two "Documentation" headings, so you couldn't link to that section
• Put "Code structure" under "Developing"
• Moved "Releases" out of "Developing"
• Made a new heading for "Working on Vite and other dependencies" since it's not necessarily related to running the tests
• Made a new section for project-specific stuff related to sending PRs

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[bluwy]

LGTM. Maybe it's worth extracting this into a CONTRIBUTING.md file, but having in the readme is fine by me as well.

[benmccann]

yeah, fair question. for now, I'll leave that for another day