Documentation: add Linux tracing to Sphinx TOC tree

This just add a index.rst for trace subsystem. More docs will
be added later.

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

Documentation/index.rst

@@ -64,6 +64,7 @@ merged much easier.
    dev-tools/index
    doc-guide/index
    kernel-hacking/index
+   trace/index
    maintainer/index
 
 Kernel API documentation

Documentation/trace/index.rst

@@ -0,0 +1,6 @@
+==========================
+Linux Tracing Technologies
+==========================
+
+.. toctree::
+   :maxdepth: 2