doc: Converted NMI-RCU.txt to NMI-RCU.rst.

This patch converts NMI-RCU from txt to rst format.
Also adds NMI-RCU in the index.rst file.

Signed-off-by: Madhuparna Bhowmik <madhuparnabhowmik04@gmail.com>
[ paulmck: Apply feedback from Phong Tran. ]
Tested-by: Phong Tran <tranmanphong@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>