docs: describe how to quickly build a trimmed kernel

Add a text explaining how to quickly build a kernel, as that's something
users will often have to do when they want to report an issue or test
proposed fixes. This is a huge and frightening task for quite a few
users these days, as many rely on pre-compiled kernels and have never
built their own. They find help on quite a few websites explaining the
process in various ways, but those howtos often omit important details
or make things too hard for the 'quickly build just for testing' case
that 'localmodconfig' is really useful for. Hence give users something
at hand to guide them, as that makes it easier for them to help with
testing, debugging, and fixing the kernel.

To keep the complexity at bay, the document explicitly focuses on how to
compile the kernel on commodity distributions running on commodity
hardware. People that deal with less common distributions or hardware
will often know their way around already anyway.

The text describes a few oddities of Arch and Debian that were found by
the author and a few volunteers that tested the described procedure.
There are likely more such quirks that need to be covered as well as a
few things the author will have missed -- but one has to start
somewhere.

The document heavily uses anchors and links to them, which makes things
slightly harder to read in the source form. But the intended target
audience is way more likely to read rendered versions of this text on
pages like docs.kernel.org anyway -- and there those anchors and links
allow easy jumps to the reference section and back, which makes the
document a lot easier to work with for the intended target audience.

Aspects relevant for bisection were left out on purpose, as that is a
related, but in the end different use case. The rough plan is to have a
second document with a similar style to cover bisection. The idea is to
reuse a few bits from this document and link quite often to entries in
the reference section with the help of the anchors in this text.

Signed-off-by: Thorsten Leemhuis <linux@leemhuis.info>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://lore.kernel.org/r/1a788a8e7ba8a2063df08668f565efa832016032.1678021408.git.linux@leemhuis.info
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/admin-guide/index.rst

@@ -37,6 +37,7 @@ problems and bugs in particular.
    reporting-issues
    reporting-regressions
    security-bugs
+   quickly-build-trimmed-linux
    bug-hunting
    bug-bisect
    tainted-kernels

MAINTAINERS

@@ -6202,6 +6202,7 @@ DOCUMENTATION REPORTING ISSUES
 M:	Thorsten Leemhuis <linux@leemhuis.info>
 L:	linux-doc@vger.kernel.org
 S:	Maintained
+F:	Documentation/admin-guide/quickly-build-trimmed-linux.rst
 F:	Documentation/admin-guide/reporting-issues.rst
 
 DOCUMENTATION SCRIPTS