doxygen build currently broken in CI runs

[edwardhartnett]

The CI runs are failing due to the doxygen build.

So WARN_AS_ERROR should not be turned on yet in the Doxyfile. I will fix.

[ 39%] Built target doxygen_doc
[ 40%] Generating API Documentation with Doxygen
warning: tag INPUT: input source '/home/runner/work/fv3atm/fv3atm/fv3atm/doc/user_guide.md' does not exist
warning: tag INPUT: input source '/home/runner/work/fv3atm/fv3atm/fv3atm/doc/2D-decomp.md' does not exist
warning: tag INPUT: input source '/home/runner/work/fv3atm/fv3atm/fv3atm/sorc/ncep_post.fd' does not exist
error: source '/home/runner/work/fv3atm/fv3atm/fv3atm/doc/user_guide.md' is not a readable file or directory... skipping.
 (warning treated as error, aborting now)

[edwardhartnett]

OK, I have a fix for this, and it causes all the CI builds to pass, which is nice.

The problem happening above was because UPP had the same CMake ENABLE_DOCS option. So to get around that, I change the fv3atm option to ENABLE_FV3ATM_DOCS. Now the UPP documentation build is not involved and things work.

I will put up a PR.