Revert "docs: Update edx.rtd links to docs.openedx.org"

[sarina]

These changes were made to conform openedx-org repo to not edx.org-specific strings. However this was done before the understanding that the help-tokens package existed, which gave the facility to switch between docs books - I'm on the fence about this revert (see discussion ) but this may be the right thing to do.

As far as merging goes there are some options:

1. Decide to close this PR and fix forward with the ability to customize docs links in some sort of configuration or plugin;
2. Merge this PR after Teak is cut and then add customization for Ulmo;
3. Merge this PR now, backport this commit to Teak, and then add customization for Ulmo

I'm making this PR so that it's ready to go in case consensus is route 3.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.68%. Comparing base (9002f7a) to head (f1ba72c).
Report is 28 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1844   +/-   ##
=======================================
  Coverage   93.68%   93.68%           
=======================================
  Files        1142     1142           
  Lines       23456    23456           
  Branches     4942     4943    +1     
=======================================
  Hits        21974    21974           
  Misses       1414     1414           
  Partials       68       68           

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