Update gitlab-markup to 1.7.0 and document Python3 requirements

We've been running docutils to render reStructuredText markup with Python 3 by patching current gitlab-markup gem in Omnibus GitLab (see original merge request: omnibus-gitlab!968 (merged))

This patch is no longer needed as we merged Python 3 support natively in gitlab-markup!9 (merged).

We need to update the gem in our codebase and remove the patch from omnibus-gitlab. Additionally we should document our python requirements and how to run it with GitLab in development.