add documentation check to CI run, ensure no documentation warnings with WARN_AS_ERROR

[edwardhartnett]

Doxygen has a WARN_AS_ERROR setting.

This setting, when turned on, cause the documentation build to error out of there are any warnings.

Once existing warnings are cleared, this can be turned on, and a doxygen build added to the CI. After that, no PR will pass if it generates doxygen warnings. This is a great way to keep documentation working and the quality of doxygen good. Of course, it does not replace the review of documentation to ensure it makes sense. ;-)

But it is very helpful, we do it now with many NOAA repositories and it works well.