trace doc: convert trace/ftrace.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: NChangbin Du <changbin.du@intel.com>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>

Documentation/trace/index.rst

@@ -7,4 +7,5 @@ Linux Tracing Technologies
 
    ftrace-design
    tracepoint-analysis
+   ftrace
    ftrace-uses