trace doc: convert trace/stm.txt to rst format

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

Cc: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Changbin Du <changbin.du@intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/trace/index.rst

@@ -20,3 +20,4 @@ Linux Tracing Technologies
    mmiotrace
    hwlat_detector
    intel_th
+   stm