Documentation: best practices for using Link trailers

Based on multiple conversations, most recently on the ksummit mailing
list [1], add some best practices for using the Link trailer, such as:

- how to use markdown-like bracketed numbers in the commit message to
indicate the corresponding link
- when to use lore.kernel.org vs patch.msgid.link domains

Cc: ksummit@lists.linux.dev
Link: https://lore.kernel.org/20240617-arboreal-industrious-hedgehog-5b84ae@meerkat

 # [1]
Signed-off-by: Konstantin Ryabitsev <konstantin@linuxfoundation.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20240619-docs-patch-msgid-link-v2-2-72dd272bfe37@linuxfoundation.org