Test Sphinx references used in our documentation

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 91.17647% with 3 lines in your changes missing coverage. Please review.

Project coverage is 96.20%. Comparing base (0d8ed5f) to head (f6a7abe). Report is 275 commits behind head on 0.4.x.

Files with missing lines	Patch %	Lines
betty/tests/test_documentation.py	91.17%	2 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## 0.4.x #1812 +/- ## ========================================== + Coverage 96.17% 96.20% +0.02% ========================================== Files 259 259 Lines 17971 17977 +6 Branches 3504 3506 +2 ========================================== + Hits 17284 17294 +10 + Misses 468 464 -4 Partials 219 219 ```

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