docs-rst: doc-guide: split the kernel-documentation.rst contents

Having the kernel-documentation at the topmost level doesn't
allow generating a separate PDF file for it. Also, makes harder
to add extra contents. So, place it on a sub-dir.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/kernel-doc-nano-HOWTO.txt

@@ -1,5 +1,5 @@
 NOTE: this document is outdated and will eventually be removed.  See
-Documentation/kernel-documentation.rst for current information.
+Documentation/doc-guide/ for current information.
 
 kernel-doc nano-HOWTO
 =====================