Add breathe bridge doxygen to sphinx

[gassmoeller]

Use breathe (https://breathe.readthedocs.io/en/latest/) to include the doxygen documentation into the sphinx documentation. Currently this doesnt look very good, because it includes all doxygen documentation on a single page.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (23b3b5e) 93.54% compared to head (353bfbb) 93.54%.
Report is 24 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #546   +/-   ##
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  Coverage   93.54%   93.54%           
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  Files          92       92           
  Lines        6340     6340           
=======================================
  Hits         5931     5931           
  Misses        409      409           

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[github-actions]

Benchmark	Main	Feature	Difference (99.9% CI)
Slab interpolation simple none	1.023 ± 0.004 (s=451)	1.022 ± 0.003 (s=432)	-0.2% .. -0.0%
Slab interpolation curved simple none	1.028 ± 0.003 (s=416)	1.026 ± 0.003 (s=463)	-0.3% .. -0.1%
Spherical slab interpolation simple none	1.259 ± 0.005 (s=358)	1.260 ± 0.009 (s=359)	-0.0% .. +0.2%
Slab interpolation simple curved CMS	1.098 ± 0.003 (s=408)	1.098 ± 0.005 (s=414)	-0.1% .. +0.1%
Spherical slab interpolation simple CMS	1.858 ± 0.006 (s=254)	1.857 ± 0.005 (s=233)	-0.2% .. +0.0%
Spherical fault interpolation simple none	1.271 ± 0.005 (s=346)	1.271 ± 0.005 (s=364)	-0.1% .. +0.1%
Cartesian min max surface	2.360 ± 0.015 (s=186)	2.361 ± 0.021 (s=198)	-0.2% .. +0.3%
Spherical min max surface	7.334 ± 0.017 (s=62)	7.338 ± 0.015 (s=63)	-0.1% .. +0.2%

[gassmoeller]

This is as good as I can make this right now. I can build the doxygen documentation into breathe, but there are doxygen warnings that are interpreted as errors by readthedocs (so it works locally, but not for the online documentation). If someone wants to work on the errors or improve the design feel free to take a stab, otherwise this PR is just for future reference.