docs: Hook the RTLA documents into the kernel docs build

The RTLA documents were added to Documentation/ but never hooked into the
rest of the docs build, leading to a bunch of warnings like:

  Documentation/tools/rtla/rtla-osnoise.rst: WARNING: document isn't included in any toctree

Add some basic glue to wire these documents into the build so that they are
available with the rest of the rendered docs.  No attempt has been made to
turn the RTLA docs into proper RST files rather than warmed-over man pages;
that is an exercise for the future.

Fixes: d40d48e1

 ("rtla: Add Documentation")
Acked-by: Daniel Bristot de Oliveira <bristot@kernel.org>
Acked-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Link: https://lore.kernel.org/r/877dau555q.fsf@meer.lwn.net


Signed-off-by: Jonathan Corbet <corbet@lwn.net>