Utilities/Sphinx: Add SPHINX_INFO option to build and install Info manual

This generates the manual in the Info format and installs it into info/cmake.info.

I have tested that it is created and installed correctly both with cmake -DSPHINX_INFO=ON and ./configure --sphinx-info on Linux.

I have kept things in alphabetical order, this is why e.g. --sphinx-info goes before --sphinx-man.

Here is the manual at the commit of this PR: cmake.info

Topic-rename: sphinx-info