Documentation: add Linux ACPI to Sphinx TOC tree

Add below index.rst files for ACPI subsystem. More docs will be added later.
  o admin-guide/acpi/index.rst
  o driver-api/acpi/index.rst
  o firmware-guide/index.rst
Signed-off-by: Changbin Du <changbin.du@gmail.com>
Reviewed-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Documentation/admin-guide/acpi/index.rst

+============
+ACPI Support
+============
+
+Here we document in detail how to interact with various mechanisms in
+the Linux ACPI support.
+
+.. toctree::
+   :maxdepth: 1
+

Documentation/admin-guide/index.rst

@@ -77,6 +77,7 @@ configure specific aspects of kernel behavior to your liking.
    LSM/index
    mm/index
    perf-security
+   acpi/index
 
 .. only::  subproject and html
 

Documentation/driver-api/acpi/index.rst

+============
+ACPI Support
+============
+
+.. toctree::
+   :maxdepth: 2
+

Documentation/driver-api/index.rst

@@ -56,6 +56,7 @@ available subsections can be seen below.
    slimbus
    soundwire/index
    fpga/index
+   acpi/index
 
 .. only::  subproject and html
 

Documentation/firmware-guide/acpi/index.rst

+.. SPDX-License-Identifier: GPL-2.0
+
+============
+ACPI Support
+============
+
+.. toctree::
+   :maxdepth: 1
+

Documentation/firmware-guide/index.rst

+.. SPDX-License-Identifier: GPL-2.0
+
+===============================
+The Linux kernel firmware guide
+===============================
+
+This section describes the ACPI subsystem in Linux from firmware perspective.
+
+.. toctree::
+   :maxdepth: 1
+
+   acpi/index
+

Documentation/index.rst

@@ -35,6 +35,16 @@ trying to get it to work optimally on a given system.
 
    admin-guide/index
 
+Firmware-related documentation
+------------------------------
+The following holds information on the kernel's expectations regarding the
+platform firmwares.
+
+.. toctree::
+   :maxdepth: 2
+
+   firmware-guide/index
+
 Application-developer documentation
 -----------------------------------