docs: remove obsolete documentation evolve-collaboration.rst replaced by sharing.rst instability.rst introduction to the concept for Mercurial developers -- no longer needed since most are now convinced obs-concept.rst largely replaced by the user guide obs-implementation.rst more for Mercurial developers; the source code is probably more accurate and up-to-date qsync.rst the extension this referred to no longer exists This fixes several more sphinx warnings. I've left some other old docs around, because they still have useful content that needs to be adapted and updated.