kernel-doc: produce RestructuredText output

If given the -rst flag, output will now be in RestructuredText.  Various
glitches to be worked out yet.

In the end I decided not to use RST section headings within the kerneldoc
comments.  gpu.tmpl already has headings five levels deep; adding more is
not going to bring clarity.

This is really just Jani Nikula's asciidoc change with the serial numbers
filed off.  It's a hack job that doubtless needs a lot of cleaning up.

Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>