docs: remove unused/obsolete figures

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.

docs: add a table of contents This silences many sphinx warnings about documents not included in any toctree. Still need to delete obsolete .rst files to silence the rest of the warnings.

merge with stable

Added tag 5.1.4 for changeset 891c3ce894fa

prepare release 5.1.4