Documentation: describe how to apply incremental stable patches

The applying patches document
(Documentation/process/applying-patches.rst) mentions incremental stable
patches, but there is no example of how to apply them. Describe the
process.

While at it, remove note about incremental patches and move the external
link of 5.x.y incremental patches to "Where can I download patches?"
section.

Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Link: https://lore.kernel.org/r/20220307063340.256671-1-bagasdotme@gmail.com


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