requested a review from a team

googlebot added the cla: yes

label

Related to: googleapis#6543

This fixes doxygen + clang being unable find certain resources like
`stddef.h` by telling clang where its `-resource-dir` is. After fixing
that error, there were a bunch of missing includes of proto/grpc
headers. Those were fixed by making the `doxygen-docs` CMake target
depend on `google-cloud-cpp-protos` so that those would be generated
before trying to generate the docs. Then some include path tweaks to
make things work.

I also enabled HTML timestamping so that we can see when new docs get
published.

This change supersedes
googleapis#6552, which I'm
goign to close.