doc(*): move most docs to website, update links (#2676)
The relaunched https://leanprover-community.github.io now contains copies of most of the doc files. This PR replaces the corresponding markdown files on mathlib with pointers to their new locations so that they only need to be maintained in one place.
The two remaining markdown files are `docs/contribute/bors.md` and `docs/contribute/code-review.md`.
Fixes #2168.
