Add backlink in docs archive to current docs

With the migration to Antora, we still wanted a way to access older versions of the documentation. This documentation is now available here:

https://vault.opennms.com/docs/

This page should have a link that will take users to the current docs and a note that this is archived documentation. Something like the following:

OpenNMS Projects: Archived Documentation

(Click this link to access the current documentation.

Link is: docs.opennms.com

Marked as blocker, as this needs to happen in tandem with merging the new antora docs.