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)

pytest >= 3.3.0, < 4.7
pytest-runner < 5.2
pytest-sugar >= 0.7.0, < 0.10
pytest-benchmark < 3.3
pytest-localserver < 0.6
mock < 3.1
Sphinx < 1.9
WebTest < 2.1
isort == 4.3.21