[Bug libstdc++/68792] Review doxygen output and don't install useless things

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68792

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
We might also want to use "MAN_EXTENSION = .3c++" in the doxyfile so that we
don't dump all these files in section 3.