Add documentation on how to redirect when moving pages

[MKKnorr]

It is a quite common occurence, that links break because pages have been moved, but no redirects added.
It should be common practice, to also add redirects when moving pages.

It seems that this mostly happens, because people are not aware of how to do this. Another reason might of course be, that not everyone has access to those settings in readthedocs.

Documenting this and providing guidelines at least creates awareness for this problem, and should help reviewers pay attention to creating the redirects before/when merging PRs that move pages.

The technical documentation for how to create redirects in readthedocs is available here: https://docs.readthedocs.io/en/stable/guides/redirects.html