Indicate if a guide link is back-level

[holly-cummins]

Sometimes (for example, with https://quarkus.io/extensions/org.apache.camel.quarkus/camel-quarkus-atlasmap), the latest released version of an extension is much newer than the latest available documentation. When that happens we do a nifty mapping to find the most recent valid documentation, but we should put some sort of visual indication that we're linking to old doc. Often this happens when an extension has been deprecated, so it's useful for users to know to be cautious.