documentation: Add documentation for RCU's major data structures

This commit adds documentation for RCU's major data structures,
including rcu_state, rcu_node, rcu_data, rcu_dynticks, and rcu_head.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

Documentation/RCU/Design/Data-Structures/TreeMapping.svg

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