Section-level re-structures

What this does

Remove the "common" Reference section in favor or persona-scoped Reference (as everything else)
- TLDR: use staging_docs/{persona}/reference instead of the global staging_docs/reference.
- I've suggested a common reference before because I was unsure how reference could be persona-scoped. After seeing more real content, I'm convinced that we should scope them for both consistency and convenience.
Re-organize Section overviews (user,admin,dev manuals)
- Move responsibility for those from pulpcore to pulp-docs, because these "sections content" are highly coupled with the navigation that pulp-docs define.
- Add some fancy tiles for most useful links/paths
- Add "Quick Links" table (auto-generated) for each of them with version and links for rest-api, github and changelog. I've found myself wishing for this sometimes, and I hope it will be generally useful.
Re-organize "Help" Section.
- Move changelogs there. It is not quite the right place, but I guess its reasonable, as we have "Quick Links" table now. An alternative is move it to its own top-level section (as I've removed "Reference" from there, it has some "space left").
- Move "Get Involved", "Code of Conduct" and "Privacy Policy" from main Pulp site here. I've also re-written "Get Involved" so its more succint.
- Add placeholder Tutorial (probably) called "Documentation Usage", for explaining the foundation concepts of the content organizations and the website structure and how to leverage them to find things.

This is similar to user and admin.