Add canonical URL links to older doc versions

When searching for terms like "Debezium snapshotting MySQL", many search results point to older doc versions instead of the latest. This leads users to finding information not 100% relevant for them, or worse, into using outdated Debezium versions. To address this, we should:

• Publish the current dev docs (1.7 at this point) under https://debezium.io/documentation/reference/dev
• Publish the current stable docs (1.6 at this point) under https://debezium.io/documentation/reference
• Have canonical URL links on all published docs other than stable, pointing to stable

As we don't tend to remove specific pages, having each page point to its counter-part under stable should be a good starting point. The recent SMT docs split-up might be a special case to think about. One other issue to consider is new pages added under dev, which are not available in stable yet.

While at it, let's also add the documentation version to the page title (e.g. "Topic Routing :: Debezium Documentation 1.7"), so it is shown in search results.