Revert "convert more .rst files to .md"

This reverts commit b7967f52c1b70aa0c7c297633438fafbfd23164a.

This commit broke CI :-(

Submitting this reversion TBR

Reviewed at https://rbcommons.com/s/twitter/r/1235/
Sync of https://github.com/pantsbuild/pants/commit/b781ac26f8bbbe26cdf0ce4845a56f7ea73a2a62