ci: speed up publish-docs build (#7657)
ci: speed up publish-docs build (#7657)

Do not scan the `*.cc` files for Doxygen comments. I searched, and the
existing Doxygen comments for such files are in files excluded for other
reasons (tests, internal symbols, etc.)  This reduced the build time
from 440s to 270s.
googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp