Update sphinx multimethod customization to handle toplevel functions

[lorenzncode]

Fixes #1921

[lorenzncode]

solid example

current docs: https://cadquery.readthedocs.io/en/latest/classreference.html#cadquery.occ_impl.shapes.solid

PR: https://cadquery--1932.org.readthedocs.build/en/1932/classreference.html#cadquery.occ_impl.shapes.solid

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.73%. Comparing base (aed5e55) to head (5d8f262).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1932      +/-   ##
==========================================
- Coverage   95.76%   95.73%   -0.03%     
==========================================
  Files          29       29              
  Lines        7833     7851      +18     
  Branches     1181     1181              
==========================================
+ Hits         7501     7516      +15     
- Misses        192      194       +2     
- Partials      140      141       +1     

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