Explore use of Sphinx as rST renderer

Due to the licensing issues with readline (see https://gitlab.com/gitlab-org/omnibus-gitlab/issues/1945), we should consider an alternate solution which does not require on-going modification or patching.

One option is Sphinx for rST rendering. Sphinx is BSD licensed which should remove any unclarities that were raised in original description of this issue.

For this do the following:

1. Bump the version of gitlab-markup gem and release it
2. Build Sphinx in omnibus
3. Compare the rendering between the two to see if we can retain feature parity