trace doc: convert trace/ftrace-design.txt to rst format

This converts the plain text documentation to reStructuredText format and
add it to Sphinx TOC tree. This documentation is not synced with current
code, so mark it as out of date.

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