Add --doc_mode config option for testing documentation images

[user27182]

Add --doc_mode for testing documentation images. This effectively moves most of the documentation tests from pyvista to pytest-pyvista.

Code is adapted from https://github.com/pyvista/pyvista/blob/main/tests/doc/tst_doc_build.py

The functionally of the tests is largely the same as the current PyVista tests except for how flaky tests are handled. Instead of having a separate flaky_tests dir, subdirectories inside the cache image dir itself are allowed, and you just place the multiple images there instead

[codecov-commenter]

Codecov Report

❌ Patch coverage is 99.09502% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.29%. Comparing base (5641141) to head (e6cb258).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
pytest_pyvista/doc_mode.py	98.88%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #198      +/-   ##
==========================================
+ Coverage   96.07%   97.29%   +1.21%     
==========================================
  Files           2        3       +1     
  Lines         306      518     +212     
  Branches       46       74      +28     
==========================================
+ Hits          294      504     +210     
  Misses          5        5              
- Partials        7        9       +2     

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[user27182]

This is ready for review. It was also tested downstream and passes CI: pyvista/pyvista#7878

[user27182]

Codecov Report

❌ Patch coverage is 6.77966% with 220 lines in your changes missing coverage. Please review. ✅ Project coverage is 17.07%. Comparing base (94813c8) to head (298011c).

Files with missing lines Patch % Lines
pytest_pyvista/doc_mode.py 0.00% 198 Missing ⚠️
pytest_pyvista/pytest_pyvista.py 42.10% 20 Missing and 2 partials ⚠️
Additional details and impacted files
☔ View full report in Codecov by Sentry. 📢 Have feedback on the report? Share it here.

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Coverage was something like this before #196 was merged:

❌ Patch coverage is 99.15% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 98.66%. Comparing base (114b35a94813c8) to head (aa39b4e329acdf).

[user27182]

@akaszynski Downstream doc build tests are green again:
https://github.com/pyvista/pyvista/actions/runs/17307258755/job/49259848236
https://github.com/pyvista/pyvista/actions/runs/17307258755/job/49260708895

[akaszynski]

A few nitpicks, otherwise LGTM.