[python] Iterate on spatial + readthedocs

[johnkerl]

Issue and/or context: #3606 was nice, but, readthedocs failed at https://app.readthedocs.org/projects/tiledbsoma/builds/26927429/ with

/home/docs/checkouts/readthedocs.org/user_builds/tiledbsoma/checkouts/latest/apis/python/src/tiledbsoma/io/spatial/outgest.py:16: UserWarning: Experimental spatial outgestor requires the spatialdata package.
  warnings.warn("Experimental spatial outgestor requires the spatialdata package.")
WARNING: Failed to import tiledbsoma.io.spatial.
...
* ModuleNotFoundError: No module named 'spatialdata'
* AttributeError: module 'tiledbsoma.io' has no attribute 'spatial'
...

Changes:

Add in the spatialdata dependency to doc/requirements_doc.txt

Notes for Reviewer:

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.31%. Comparing base (c515933) to head (3d801f5).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3608      +/-   ##
==========================================
+ 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> (∅)