Replace or remove all legacy readthedocs links in links.txt

[jswope00]

In the links.txt file, there are still references to legacy docs which can be identified by the readthedocs.io URL.

As we migrate all docs into docs.openedx.org, there shouldn't be a need to link to a legacy site anymore. So we should do likely one of two things for each of these legacy URLs.

1. Replace the external link with an internal :ref: to the internal doc, then remove the link from links.txt.
2. Adjust the doc accordingly if the legacy doc is unavailable or out of date. Remove the link from the doc and from links.txt to avoid confusion.

[sarina]

Partially addressed by #854 and #847 . Won't be complete until #833 is resolved.