Add DOCUMENTATION_URL as a standard value for /etc/os-release

Add DOCUMENTATION_URL as a standard value for /etc/os-release

It is very useful for distributions to be able to set a primary
documentation URL in a standard location so that users and
applications on the system can identify it. For example, many
headless systems these days use the "Cockpit" admin console. It
would be ideal if we could specify this location directly in the
os-release file so that any application or service could have a
well-known location for retrieving this and displaying it
appropriately. Users could likewise examine /etc/os-release to
learn this location.

Related: https://github.com/cockpit-project/cockpit/issues/10198

Signed-off-by: Stephen Gallagher <sgallagh@redhat.com>