docs: Document cross-referencing using relative path

Update the Cross-referencing section to explain how to create a
cross-reference to a document using relative paths and with no
additional syntax, by relying on automarkup.py.

Signed-off-by: Nícolas F. R. A. Prado <nfraprado@protonmail.com>
Link: https://lore.kernel.org/r/20210128010028.58541-3-nfraprado@protonmail.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>