Docs: Use CMake to build docs directly

[dbaston]

          > Would it make sense to remove the Makefile and use CMake directly to build the docs?

yes, such further enhancements are totally reasonable. I just wanted to do something relatively quick to get documentation building easily accessible from CMake. Would you like following up on this in subsequent PRs ?

(I'm wondering if we could have things not requiring a Unix environment so doc building is available from non-Cygwin Windows (provided folks have installed Doxygen, sphinx, etc.), but that would be more a stretch goal...)

Originally posted by @rouault in #9836 (comment)