doc: Add CPU Isolation documentation

doc: Add CPU Isolation documentation

nohz_full was introduced in v3.10 in 2013, which means this
documentation is overdue for 13 years.

Fortunately Paul wrote a part of the needed documentation a while ago,
especially concerning nohz_full in Documentation/timers/no_hz.rst and
also about per-CPU kthreads in
Documentation/admin-guide/kernel-per-CPU-kthreads.rst

Introduce a new page that gives an overview of CPU isolation in general.

Acked-by: Waiman Long <longman@redhat.com>
Reviewed-by: Valentin Schneider <vschneid@redhat.com>
Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <20260402094749.18879-1-frederic@kernel.org>