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: d40d48e1f1f2 ("rtla: Add Documentation")
Acked-by: Daniel Bristot de Oliveira <[e-mail]>
Acked-by: Steven Rostedt (Google) <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jonathan Corbet <[e-mail]>

Documentation/index.rst

···
 .. toctree::
    :maxdepth: 2
 
+   tools/index
    staging/index
    watch_queue
 

Documentation/tools/index.rst

···
+.. SPDX-License-Identifier: GPL-2.0
+
+============
+Kernel tools
+============
+
+This book covers user-space tools that are shipped with the kernel source;
+more additions are needed here:
+
+.. toctree::
+   :maxdepth: 1
+
+   rtla/index
+
+.. only::  subproject and html
+
+   Indices
+   =======
+
+   * :ref:`genindex`

Documentation/tools/rtla/index.rst

···
+.. SPDX-License-Identifier: GPL-2.0
+
+================================
+The realtime Linux analysis tool
+================================
+
+RTLA provides a set of tools for the analysis of the kernel's realtime
+behavior on specific hardware.
+
+.. toctree::
+   :maxdepth: 1
+
+   rtla
+   rtla-osnoise
+   rtla-osnoise-hist
+   rtla-osnoise-top
+   rtla-timerlat
+   rtla-timerlat-hist
+   rtla-timerlat-top
+
+.. only::  subproject and html
+
+   Indices
+   =======
+
+   * :ref:`genindex`