docs: doc-guide: avoid using ReST :doc:`foo` markup

The :doc:`foo` tag is auto-generated via automarkup.py.
So, use the filename at the sources, instead of :doc:`foo`.
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/d6cbe5183406e3378ed4bd0f84f4bcf85a15009c.1623824363.git.mchehab+huawei@kernel.orgSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/doc-guide/contributing.rst

@@ -237,10 +237,10 @@ We have been trying to improve the situation through the creation of
 a set of "books" that group documentation for specific readers.  These
 include:
 
- - :doc:`../admin-guide/index`
- - :doc:`../core-api/index`
- - :doc:`../driver-api/index`
- - :doc:`../userspace-api/index`
+ - Documentation/admin-guide/index.rst
+ - Documentation/core-api/index.rst
+ - Documentation/driver-api/index.rst
+ - Documentation/userspace-api/index.rst
 
 As well as this book on documentation itself.