Ensure no warnings on docs build

[tkknight]

Minor updates to the docs:

• Force docs build warnings to be errors
• Force doc build to continue if there are errors
• Updated an intersphinx mapping
• Added the :orphan: tag to the release notes file , else it raises a warning (toctree will now ignore this file)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.16%. Comparing base (0bd6351) to head (9cf0cbf).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #471   +/-   ##
=======================================
  Coverage   89.16%   89.16%           
=======================================
  Files           8        8           
  Lines        2474     2474           
  Branches      418      418           
=======================================
  Hits         2206     2206           
  Misses        170      170           
  Partials       98       98           

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[pp-mo]

This looks fine. And all useful ideas.