docs: add high-level description of SSH repository access In addition to the existing technical documentation about SSH repository access, add some high-level info about what this means. (some editing by Thomas De Schampheleire)

# 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
.