docs: doc-guide: remove the extension from .rst files

On almost all places, we're including ReST files without the
extension.

Let's remove the extension here as well, in order to use just
one standard.
Suggested-by: NJani Nikula <jani.nikula@linux.intel.com>
Signed-off-by: NMauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>

Documentation/doc-guide/index.rst

@@ -7,9 +7,9 @@ How to write kernel documentation
 .. toctree::
    :maxdepth: 1
 
-   sphinx.rst
-   kernel-doc.rst
-   parse-headers.rst
+   sphinx
+   kernel-doc
+   parse-headers
 
 .. only::  subproject and html