doc: Convert to rcubarrier.txt to ReST

Convert rcubarrier.txt to rcubarrier.rst and add it to index.rst.

Format file according to reST
- Add headings and sub-headings
- Add code segments
- Add cross-references to quizes and answers
Signed-off-by: Amol Grover <frextrite@gmail.com>
Tested-by: Phong Tran <tranmanphong@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

Documentation/RCU/index.rst

@@ -8,6 +8,7 @@ RCU concepts
    :maxdepth: 3
 
    arrayRCU
+   rcubarrier
    rcu_dereference
    whatisRCU
    rcu