build: Separate options for manpages install and gtk-doc generation

The main use case for the docs option seems to be to not spend time on gtk-doc generation. It is rather unexpected this also disables the installation of manpages, which are just installed as-is with no work involved. Option name 'gtk-doc' instead of 'gtk_doc' is used, because the option is about generating API docs with the gtk-doc tool, which has the hyphen in its name; 'gtk_doc' would mean 'gtk doc', which this is not.