license-expectations: Document scan-copyrights

Currently, Apertis CI pipeline uses scan-copyrights Perl tool to
generate and update the copyright report. However, this is not
documented anywhere in the website, only fossology is mentioned as a
future improvement.

Let's document the current scan-copyrights approach.

Signed-off-by: Ariel D'Alessandro <ariel.dalessandro@collabora.com>