Remove or link orphaned docs pages

Orphaned docs pages that are not liked from out docs either in the narrative, or any parts of it via intersphinx, should ideally need to be removed from here.

Any downstream that are linking to those pages should just update their links, we can search for those on github if needed and notice them/open PRs, too.

EDIT: xref https://github.com/astropy/astropy/pull/9727#discussion_r355628288

Start by identifying documentation pages that are not linked from the narrative or referenced through intersphinx. Search GitHub for downstream links to any candidates, then remove orphaned pages or link them appropriately and address affected downstream references.

Written by the indexing model from the issue text.