Unify repository and website documentation

[jescalada]

Currently, documentation is scattered across the repository (/docs) and website (/website/*.mdx). We should find a way to unify existing docs, perhaps by converting everything into repo docs, and then publishing those directly to the website so that both are identical.

Note that MDX allows for a few more text formatting options, so existing website docs might have to be simplified a bit when converting to vanilla Markdown.

Additional context
Related discussion: #1391

Should these docs move into /website/docs to keep it all in one place?
Also thinking about @andypols comments about separating docs in the website for end users and those configuring/deploying git proxy. The docs in this /docs folder are all useful to those deploying or upgrading, so perhaps we should take this oppurtunity to unite them and do that division?

Related issues: #1272, #1346, #1347

[kriswest]

It seems simpler to move all the documentation into /website/docs (where both .md and .mdx are supported), then to migrate website docs in the other direction. The documents in /docs are all recent additions. If moved into /website/docs they can be consumed either through the website or directly (in a checkout or in GitHub via a link in the README.md - keep them in the .md format and GitHub will render them, or link to the page in the website and let it render them).