Restore missing types in reference docs

[dbolduc]

Certain symbols (e.g. MakeGoogleDefaultCredentials()) are missing from our reference docs. The missing types have an @ingroup foo. Doxygen renders these as <member> tags, which we skip in our docfx -> yaml conversion:

https://github.com/googleapis/google-cloud-cpp/blob/4ce192283831c5f0d815b84462bb53fa294a03af/docfx/doxygen2yaml.cc#L62-L66

The docs for google-cloud-cpp version >= v2.23.0 are affected. When we have a fix, we ought to backport it to the older versions.

[dbolduc]

For example, if we (circa 2024-10-29) look at: https://cloud.google.com/cpp/docs/reference/common/2.22.0/namespacegoogle_1_1cloud

... we can see MakeAccessTokenCredentials(...) documented.

We do not see it for other versions of the library: https://cloud.google.com/cpp/docs/reference/common/latest/namespacegoogle_1_1cloud

[dbolduc]

Also, these functions are broken links: https://cloud.google.com/cpp/docs/reference/common/2.31.0/group__guac#functions