Man page updates and improvements

The gnome-shell man page got horribly outdated over time - it misses all the wayland-related options, still references bugzilla etc.

I suspect that using docbook XML as the source format is partly to blame, so move the page to asciidoc before bringing it up to date.

Asciidoc is already used by the extensions-tool, but some distros have raised concerns about the dependency. Address that as well by including the generated man pages in the tarball, so the dependency is only needed when building from git.