Fix Sphinx warnings

[bartfeenstra]

No description provided.

[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 98.56%. Comparing base (34ef19d) to head (a03ad54).
⚠️ Report is 2 commits behind head on 0.5.x.

Files with missing lines	Patch %	Lines
betty/documentation.py	80.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##            0.5.x    #3443      +/-   ##
==========================================
- Coverage   98.57%   98.56%   -0.01%     
==========================================
  Files         592      592              
  Lines       31013    31022       +9     
  Branches     1276     1277       +1     
==========================================
+ Hits        30570    30577       +7     
- Misses        299      300       +1     
- Partials      144      145       +1     

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