Add banner to older doc versions about them being outdated

Similar to what's done in Hibernate or OpenShift docs, we should have some sort of banner or label on older published doc versions, saying "You are viewing documentation for a release that is no longer maintained. The documentation for the current stable release of Debezium can be found at ...". A bit nicer looking than the OpenShift one, though

Related to that, would also be interesting to get an update from Antora folks, whether they now support a) a "stable" alias for the current docs version and b) canonical links on older docs versions referring to the current stable one.