[doc] Fix RTD build + other fixes/improvement of the doc

[simogasp]

RTD was not showing the correct index.html page because sphinx overwrites the index.html generated by doxygen and imported in the sphinx build. The workaround is to make sphinx generate the HTML files with a custom extension so that the doxygen one is not overwritten.

Other than that

• Major review of the README.md for the camera module, some formulas were not correct.
• Minor fixes for other READMEs, mostly format and maquillage

Now the doc seems to be working again https://alicevision.readthedocs.io/en/doc-fixes/

⚠️ There are still open issues though:

1. The math equations do not work because doxygen do no recognize the syntax in markdown e.g. https://alicevision.readthedocs.io/en/doc-fixes/md__home_docs_checkouts_readthedocs_8org_user_builds_alicevision_checkouts_doc-fixes_src_aliceVision_camera_README.html One solution could be to use \f instead of $ that should work but then it won't show in markdown... (this is the opencv solution)
2. there is still the problem with the images that do not show because of the relative path (there is a ../ missing but it won't then show the image in the markdown), e.g. https://alicevision.readthedocs.io/en/doc-fixes/md__home_docs_checkouts_readthedocs_8org_user_builds_alicevision_checkouts_doc-fixes_src_aliceVision_camera_README.html