Documentation: add meta-documentation for Sphinx and kernel-doc

Describe Sphinx, reStructuredText, the kernel-doc extension, the
kernel-doc structured documentation comments, etc.

The kernel-doc parts are based on kernel-doc-nano-HOWTO.txt, by Tim
<twaugh@redhat.com>.

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