MAINTAINERS: Add doc files on real-time support to Real-time Linux

MAINTAINERS: Add doc files on real-time support to Real-time Linux

Commit f51fe3b7e48c ("Documentation: Add real-time to core-api") adds new
documentation on real-time support, i.e., PREEMPT_RT. So, add a file entry
for that directory to the corresponding section in MAINTAINERS.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@redhat.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <20251127154343.292156-4-bigeasy@linutronix.de>