docs: add two documents about regression handling

Create two documents explaining various aspects around regression
handling and tracking; one is aimed at users, the other targets
developers.

The texts among others describes the first rule of Linux kernel
development and what it means in practice. They also explain what a
regression actually is and how to report one properly.

Both texts additionally provide a brief introduction to the bot the
kernel's regression tracker uses to facilitate the work, but mention the
use is optional.

To sum things up, provide a few quotes from Linus in the document for
developers to show how serious we take regressions.
Signed-off-by: Thorsten Leemhuis <linux@leemhuis.info>
Link: https://lore.kernel.org/r/34e56d3588f22d7e0b4d635ef9c9c3b33ca4ac04.1644994117.git.linux@leemhuis.infoSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/admin-guide/index.rst

@@ -35,6 +35,7 @@ problems and bugs in particular.
    :maxdepth: 1
 
    reporting-issues
+   reporting-regressions
    security-bugs
    bug-hunting
    bug-bisect

Documentation/process/index.rst

@@ -25,6 +25,7 @@ Below are the essential guides that every developer should read.
    code-of-conduct-interpretation
    development-process
    submitting-patches
+   handling-regressions
    programming-language
    coding-style
    maintainer-handbooks

MAINTAINERS

@@ -10438,6 +10438,8 @@ KERNEL REGRESSIONS
 M:	Thorsten Leemhuis <linux@leemhuis.info>
 L:	regressions@lists.linux.dev
 S:	Supported
+F:	Documentation/admin-guide/reporting-regressions.rst
+F:	Documentation/process/handling-regressions.rst
 
 KERNEL SELFTEST FRAMEWORK
 M:	Shuah Khan <shuah@kernel.org>