Documentation: tty: n_gsm, use power of ReST

Reformat the whole document, so that automatic numbering and TOC is
properly generated.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20220411110143.10019-7-jslaby@suse.cz


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>