docs: split vcs_support into admin/vcs_setup and usage/vcs_notes The existing page on VCS support was a mix of information needed to setup Kallithea with respect to version control systems, with information regarding using version control systems (or specific aspects of it) with Kallithea. Move the first part to the Administrator Guide, and rebrand the second part as VCS Usage Notes. In vcs_notes.rst, the general info is moved above the Mercurial-specific part, but otherwise left untouched.

# requirements.txt file for use as "pip install -r requirements.txt" as a
# readthedocs compatible alternative to "pip install -e ." which is a working
# alternative to "setup.py develop" which doesn't work with Mercurial 3.7
.