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.