Documentation: x86: convert x86_64/boot-options.txt to reST

This converts the plain text documentation to reStructuredText format and
add it to Sphinx TOC tree. No essential content change.
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/x86/index.rst

@@ -27,3 +27,4 @@ x86-specific Documentation
    resctrl_ui
    usb-legacy-support
    i386/index
+   x86_64/index

Documentation/x86/x86_64/index.rst

+.. SPDX-License-Identifier: GPL-2.0
+
+==============
+x86_64 Support
+==============
+
+.. toctree::
+   :maxdepth: 2
+
+   boot-options