Documentation: RCU: Add TOC tree hooks

Add TOC tree hooks for:
  - rcu
  - listRCU
  - UP
Signed-off-by: Jiunn Chang <c0d1n61at3@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/RCU/index.rst

+.. _rcu_concepts:
+
+============
+RCU concepts
+============
+
+.. toctree::
+   :maxdepth: 1
+
+   rcu
+   listRCU
+   UP
+
+.. only:: subproject and html
+
+   Indices
+   =======
+
+   * :ref:`genindex`