Style non-xref links differently

We should give some visual indication that a link will take you to a non-Docs domain. For example, when I use link text for something like, "Download the Couchbase image from Docker Hub.", the link styling should not be the exact same as a traditional xref to another Docs page. It helps set expectations before the user clicks, to avoid disappointment, and to prevent anxiety about breaking train of thought.

I think this could be best accomplished by having Antora automatically insert an inline image next to every HTTP link that leads to a non-Docs domain. The image should have very light styling, providing a very minor visual breadcrumb.

Mongo does this, but they're inconsistent with its usage. They seem to haphazardly use it for internal and external links.  Also, I feel like the icon is too big. We should collect more examples.

• https://docs.mongodb.com/manual/
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