docs: add backporting and conflict resolution document

This is a new document based on my 2022 blog post:

  https://blogs.oracle.com/linux/post/backporting-patches-using-git

Although this is aimed at stable contributors and distro maintainers,
it does also contain useful tips and tricks for anybody who needs to
resolve merge conflicts.

By adding this to the kernel as documentation we can more easily point
to it e.g. from stable emails about failed backports, as well as allow
the community to modify it over time if necessary.

I've added this under process/ since it also has
process/applying-patches.rst. Another interesting document is
maintainer/rebasing-and-merging.rst which maybe should eventually refer
to this one, but I'm leaving that as a future cleanup.

Thanks to Harshit Mogalapalli for helping with the original blog post
as well as this updated document and Bagas Sanjaya for providing
thoughtful feedback.

v2: fixed heading style, link style, placeholder style, other comments

Link: https://lore.kernel.org/linux-doc/20230303162553.17212-1-vegard.nossum@oracle.com/
Cc: Harshit Mogalapalli <harshit.m.mogalapalli@oracle.com>
Cc: Bagas Sanjaya <bagasdotme@gmail.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: "Jason A. Donenfeld" <Jason@zx2c4.com>
Cc: Konstantin Ryabitsev <konstantin@linuxfoundation.org>
Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>
Rule: stable@vger.kernel.org'or'commit
Link: https://lore.kernel.org/stable/20230824092325.1464227-1-vegard.nossum%40oracle.comSigned-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20230824092325.1464227-1-vegard.nossum@oracle.com

Documentation/process/index.rst

@@ -66,6 +66,7 @@ lack of a better place.
    :maxdepth: 1
 
    applying-patches
+   backporting
    adding-syscalls
    magic-number
    volatile-considered-harmful