[python] Add spatial I/O functions to readthedocs

[jp-dark]

Issue and/or context: #3592

Changes:
Add the tiledbsoma.io.spatial module and the ExperimentQueryAxis.to_spatialdata method to read the docs.

Notes for Reviewer:

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.31%. Comparing base (422b1f0) to head (02f39d7).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3595      +/-   ##
==========================================
+ Coverage   86.26%   86.31%   +0.04%     
==========================================
  Files          55       55              
  Lines        6378     6378              
==========================================
+ Hits         5502     5505       +3     
+ Misses        876      873       -3     

Flag	Coverage Δ
python	86.31% <ø> (+0.04%)	⬆️

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Components	Coverage Δ
python_api	86.31% <ø> (+0.04%)	⬆️
libtiledbsoma	∅ <ø> (∅)