Improve the documentation builds, including API docs

[marchdf]

Summary

Improve the documentation builds, including API docs. Now we can build all the documentation with just

$ cd build && cmake -DAMR_WIND_ENABLE_DOCUMENTATION:BOOL=ON .. && cmake --build . -t docs

The API documentation, instead of being at a separate site and looking distinctly different, it is embedded in our usual sphinx documentation. Also update the doxyfile to a newer doxygen.

Before: 2 websites

First site

Second site

Now: 1 website with embedded API docs using a similar style.

Pull request type

Please check the type of change introduced:

• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation content changes
• Other (please describe):

Checklist

The following is included:

• new unit-test(s)
• new regression test(s)
• documentation for new capability

[marchdf]

This apparently also makes us see all the doxygen warnings. So yay, I will fix that too.