Documentation/process: Add maintainer handbooks section

General rules for patch submission, coding style and related details are
available, but most subsystems have their subsystem-specific extra rules
which differ or go beyond the common rules.

Mark suggested to add a subsystem/maintainer handbook section, where
subsystem maintainers can explain their specific quirks.

Add the section and link to it from the submitting-patches document.

  [ bp: Add a SPDX identifier. ]
Suggested-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20181107171149.074948887@linutronix.de
Link: https://lore.kernel.org/r/20210913153942.15251-2-bp@alien8.deSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/process/index.rst

@@ -27,6 +27,7 @@ Below are the essential guides that every developer should read.
    submitting-patches
    programming-language
    coding-style
+   maintainer-handbooks
    maintainer-pgp-guide
    email-clients
    kernel-enforcement-statement

Documentation/process/maintainer-handbooks.rst

+.. SPDX-License-Identifier: GPL-2.0
+
+.. _maintainer_handbooks_main:
+
+Subsystem and maintainer tree specific development process notes
+================================================================
+
+The purpose of this document is to provide subsystem specific information
+which is supplementary to the general development process handbook
+:ref:`Documentation/process <development_process_main>`.
+
+Contents:
+
+.. toctree::
+   :numbered:
+   :maxdepth: 2

Documentation/process/submitting-patches.rst

@@ -21,6 +21,10 @@ If you're unfamiliar with ``git``, you would be well-advised to learn how to
 use it, it will make your life as a kernel developer and in general much
 easier.
 
+Some subsystems and maintainer trees have additional information about
+their workflow and expectations, see :ref:`Documentation/process/maintainer
+handbooks <maintainer_handbooks_main>`.
+
 Obtain a current source tree
 ----------------------------