docs: deprecate kernel-doc-nano-HOWTO.txt

Now that the new Sphinx world order is taking over, the information in
kernel-doc-nano-HOWTO.txt is outmoded.  I hate to remove it altogether,
since it's one of those files that people expect to find.  But we can add a
warning and fix all the other pointers to it.

Reminded-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>