Remove or link orphaned docs pages

[bsipocz]

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

[Ayoob7]

Hi I would like to work on this? any possible pointers on where to begin please ?

[tactipus]

I would also like to work on this issue.