move all documentation-specific processing to Sphinx extension

[ilia-kats]

Currently, everything is done at import time, including docstring parsing and processing. Some code also has different branches depending on whether we are currently building documentation or not to enable nice-looking internal cross-references.

This PR moves all of that to a custom Sphinx extension. This has several advantages:

• We can use a proper docstring parsing library instead of our custom, admittedly brittle, code, without making it a runtime dependency.
• faster import, since we don't need to do all that string processing at import time.
• cleaner code: The core only handles what it needs to do for the dynamic API, all documentation-specific stuff is separate, instead of being sprinkled all over core.

[codecov]

Codecov Report

❌ Patch coverage is 98.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 90.58%. Comparing base (78e5079) to head (2ed9eed).

Files with missing lines	Patch %	Lines
src/mofaflex/_core/api/utils.py	96.42%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #203      +/-   ##
==========================================
+ Coverage   90.20%   90.58%   +0.37%     
==========================================
  Files          56       56              
  Lines        5811     5745      -66     
==========================================
- Hits         5242     5204      -38     
+ Misses        569      541      -28     

Files with missing lines	Coverage Δ
src/mofaflex/_core/api/_generate.py	87.03% <100.00%> (+2.29%)	⬆️
src/mofaflex/_core/api/priors.py	100.00% <100.00%> (+7.69%)	⬆️
src/mofaflex/_core/api/terms.py	95.45% <100.00%> (+13.31%)	⬆️
src/mofaflex/_core/api/types.py	100.00% <100.00%> (+21.05%)	⬆️
src/mofaflex/_core/priors/__init__.py	100.00% <100.00%> (ø)
src/mofaflex/_core/priors/spike_slab.py	92.70% <ø> (ø)
src/mofaflex/_core/terms/mofaflex.py	94.36% <100.00%> (+2.24%)	⬆️
src/mofaflex/_core/utils.py	93.89% <ø> (-0.34%)	⬇️
src/mofaflex/tl/_downstream.py	92.85% <100.00%> (+0.17%)	⬆️
src/mofaflex/_core/api/utils.py	94.62% <96.42%> (-0.12%)	⬇️

... and 1 file with indirect coverage changes

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