Documentation/process: Add maintainer handbooks section

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.de
Signed-off-by: Jonathan Corbet <corbet@lwn.net>