Documentation: add Linux x86 docs to Sphinx TOC tree

Add a index.rst for x86 support. More docs will be added later.
Signed-off-by: Changbin Du <changbin.du@gmail.com>
Reviewed-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/index.rst

@@ -101,6 +101,7 @@ implementation.
 .. toctree::
    :maxdepth: 2
 
+   x86/index
    sh/index
 
 Filesystem Documentation

Documentation/x86/index.rst

+.. SPDX-License-Identifier: GPL-2.0
+
+==========================
+x86-specific Documentation
+==========================
+
+.. toctree::
+   :maxdepth: 2
+   :numbered: