Documentation: x86: convert x86_64/cpu-hotplug-spec 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/x86_64/cpu-hotplug-spec → Documentation/x86/x86_64/cpu-hotplug-spec.rst

+.. SPDX-License-Identifier: GPL-2.0
+
+===================================================
 Firmware support for CPU hotplug under Linux/x86-64
----------------------------------------------------
+===================================================
 
 Linux/x86-64 supports CPU hotplug now. For various reasons Linux wants to
 know in advance of boot time the maximum number of CPUs that could be plugged

Documentation/x86/x86_64/index.rst

@@ -12,3 +12,4 @@ x86_64 Support
    mm
    5level-paging
    fake-numa-for-cpusets
+   cpu-hotplug-spec