(Closes #3291) update docs for LFRic builtin map

[arporter]

No description provided.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.95%. Comparing base (2a1efa4) to head (72485d0).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3331   +/-   ##
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  Coverage   99.95%   99.95%           
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  Files         381      381           
  Lines       54015    54015           
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  Hits        53993    53993           
  Misses         22       22           

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[arporter]

Simply documents the BUILTIN_MAP property as the way to find out the names of Builtins. Also corrects some of the paths used in doctests (since the docs are now built from the doc directory) and removes an outdated UML picture.
One for @LonelyCat124 or @sergisiso.