doc: Convert to rcu_dereference.txt to rcu_dereference.rst

This patch converts rcu_dereference.txt to rcu_dereference.rst and
adds it to index.rst

Signed-off-by: Amol Grover <frextrite@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
2 files changed