Currently, all our docs are written in (github-flavored) markdown. There have been two attempts to write RFCs or docs in RestructuredText (ReST or .rst) (#18575 and #18977); this issue will serve as the decision point to either

1. Reiterate our use of markdown and insist on it for all docs/rfcs
2. Allow both markdown and ReST for new docs according to the author's preference
3. Make ReST the preferred format for new docs (possibly also migrating existing docs)

If we start using ReST, we should also standardize on a toolchain (#18977 uses docutils and graphviz and some ad-hoc scripts).

cockroachdb/docs#1716 discusses moving the docs repo away from markdown; it would be nice to move in the same direction here.