docs: remove some index.rst cruft

There is some useless boilerplate text that was added by sphinx when this
file was first created; take it out.
Reviewed-by: David Vernet <void@manifault.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Link: https://lore.kernel.org/r/20220927160559.97154-5-corbet@lwn.netSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/index.rst

 .. SPDX-License-Identifier: GPL-2.0
 
-
-.. The Linux Kernel documentation master file, created by
-   sphinx-quickstart on Fri Feb 12 13:51:46 2016.
-   You can adapt this file completely to your liking, but it should at least
-   contain the root `toctree` directive.
-
 .. _linux_doc:
 
 The Linux Kernel documentation