Documentation: x86: convert resctrl_ui.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

@@ -23,3 +23,4 @@ x86-specific Documentation
    amd-memory-encryption
    pti
    microcode
+   resctrl_ui