docs: rcu: Replace multiple dead OLS links in RTFP.txt

This patch updates several dead OLS links in RTFP.txt, replacing them
with working copies hosted on kernel.org.

Originally posted as part of a 2-patch series, this is now being sent
as a standalone v4 patch to avoid confusion.

Signed-off-by: Nikil Paul S <snikilpaul@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>