Documentation: RCU: Reduce toctree depth

toctree index for RCU currently has maximum depth of 3. Since no docs
currently use more than 3 section heading levels, this effectively spills
the entire docs hierarchy.

Tidy up by reducing toctree depth to 2 (only showing docs title and
second-level sections).

Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>