Incorporate Doxygen docs into the new website.

[brtnfld]

Using Doxygen+Breathe+Sphinx, generate the MLL and other API documentation and include it in the website.

[brtnfld]

@MicK7, I've completed the C MLL Doxygen into Read-the-docs. I created a docs directory in the CGNS repo and processed it in-place: https://github.com/brtnfld/CGNS/tree/breathe. If interested, you can build it by doing "make html" in the docs directory. You can view the results from _build/html/index/html

I'm wondering if we should add the CGNS source as a submodule in this repo and process it as part of an action or generate the webpage in the CGNS repository and reference it from this repo. What are your thoughts on the best way to do this?

It would be nice to version the documentation generated with a released version of CGNS.

Getting the C MLL into the website as proof of concept would be good before proceeding to the other documentation.