docs: add IRQ documentation at the core-api book

There are 4 IRQ documentation files under Documentation/*.txt.

Move them into a new directory (core-api/irq) and add a new
index file for it.

While here, use a title markup for the Debugging section of the
irq-domain.rst file.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/2da7485c3718e1442e6b4c2dd66857b776e8899b.1588345503.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>