docs: Update outdated edx.rtd links to docs.openedx.org

[sarina]

Ref openedx/docs.openedx.org#955

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.18%. Comparing base (6284e71) to head (2dc4267).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #400   +/-   ##
=======================================
  Coverage   99.18%   99.18%           
=======================================
  Files          75       75           
  Lines        1711     1711           
  Branches      414      412    -2     
=======================================
  Hits         1697     1697           
  Misses         14       14           

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