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 <[e-mail]>
Link: https://lore.kernel.org/r/2da748[e-mail]
Signed-off-by: Jonathan Corbet <[e-mail]>

Documentation/IRQ-domain.txt Documentation/core-api/irq/irq-domain.rst

···
 Hierarchy irq_domain is in no way x86 specific, and is heavily used to
 support other architectures, such as ARM, ARM64 etc.
 
-=== Debugging ===
+Debugging
+=========
 
 Most of the internals of the IRQ subsystem are exposed in debugfs by
 turning CONFIG_GENERIC_IRQ_DEBUGFS on.

Documentation/admin-guide/hw-vuln/l1tf.rst

···
    /proc/irq/$NR/smp_affinity[_list] files. Limited documentation is
    available at:
 
-   https://www.kernel.org/doc/Documentation/IRQ-affinity.txt
+   https://www.kernel.org/doc/Documentation/core-api/irq/irq-affinity.rst
 
 .. _smt_control:
 

Documentation/admin-guide/kernel-per-CPU-kthreads.rst

···
 References
 ==========
 
--	Documentation/IRQ-affinity.txt:  Binding interrupts to sets of CPUs.
+-	Documentation/core-api/irq/irq-affinity.rst:  Binding interrupts to sets of CPUs.
 
 -	Documentation/admin-guide/cgroup-v1:  Using cgroups to bind tasks to sets of CPUs.
 

Documentation/core-api/index.rst

···
 
    atomic_ops
    refcount-vs-atomic
+   irq/index
    local_ops
    padata
    ../RCU/index

Documentation/core-api/irq/index.rst

···
+====
+IRQs
+====
+
+.. toctree::
+   :maxdepth: 1
+
+   concepts
+   irq-affinity
+   irq-domain
+   irqflags-tracing

Documentation/ia64/irq-redir.rst

···
 
 By writing to /proc/irq/IRQ#/smp_affinity the interrupt routing can be
 controlled. The behavior on IA64 platforms is slightly different from
-that described in Documentation/IRQ-affinity.txt for i386 systems.
+that described in Documentation/core-api/irq/irq-affinity.rst for i386 systems.
 
 Because of the usage of SAPIC mode and physical destination mode the
 IRQ target is one particular CPU and cannot be a mask of several

Documentation/networking/scaling.rst

···
 an IRQ may be handled on any CPU. Because a non-negligible part of packet
 processing takes place in receive interrupt handling, it is advantageous
 to spread receive interrupts between CPUs. To manually adjust the IRQ
-affinity of each interrupt see Documentation/IRQ-affinity.txt. Some systems
+affinity of each interrupt see Documentation/core-api/irq/irq-affinity.rst. Some systems
 will be running irqbalance, a daemon that dynamically optimizes IRQ
 assignments and as a result may override any manual settings.
 
···
 
 This file implements a bitmap of CPUs. RPS is disabled when it is zero
 (the default), in which case packets are processed on the interrupting
-CPU. Documentation/IRQ-affinity.txt explains how CPUs are assigned to
+CPU. Documentation/core-api/irq/irq-affinity.rst explains how CPUs are assigned to
 the bitmap.
 
 

Documentation/translations/zh_CN/IRQ.txt

···
-Chinese translated version of Documentation/IRQ.txt
+Chinese translated version of Documentation/core-api/irq/index.rst
 
 If you have any comment or update to the content, please contact the
 original document maintainer directly.  However, if you have a problem
···
 Maintainer: Eric W. Biederman <[e-mail]>
 Chinese maintainer: Fu Wei <[e-mail]>
 ---------------------------------------------------------------------
-Documentation/IRQ.txt 的中文翻译
+Documentation/core-api/irq/index.rst 的中文翻译
 
 如果想评论或更新本文的内容，请直接联系原文档的维护者。如果你使用英文
 交流有困难的话，也可以向中文版维护者求助。如果本翻译更新不及时或者翻

MAINTAINERS

···
 M:	Marc Zyngier <[e-mail]>
 S:	Maintained
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git irq/core
-F:	Documentation/IRQ-domain.txt
+F:	Documentation/core-api/irq/irq-domain.rst
 F:	include/linux/irqdomain.h
 F:	kernel/irq/irqdomain.c
 F:	kernel/irq/msi.c