doc: Convert whatisRCU.txt to .rst

This commit updates whatisRCU.txt to the new .rst format.
This change includes:

- Formatting bullet lists
- Adding literal blocks
- Links from table of contents to corresponding sections
- Links to external documents
- Reformat quick quizzes

Signed-off-by: Phong Tran <[email protected]>
Tested-by: Madhuparna Bhowmik <[email protected]>
[ tranmanphong: Apply Amol Grover feedback. ]
Reviewed-by: Amol Grover <[email protected]>
Signed-off-by: Paul E. McKenney <[email protected]>

Author: phongt

Documentation/RCU/index.rst

@@ -8,6 +8,7 @@ RCU concepts
    :maxdepth: 3
 
    arrayRCU
+   whatisRCU
    rcu
    listRCU
    NMI-RCU