doc/rcu: Update LWN article URLs and add 2019 article

This patch adds LWN articles about RCU APIs which were released in 2019.
Also, HTTP URLs are replaced by HTTPS.

Signed-off-by: Shao-Tse Hung <ccs100203@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>