Non-xref links should open in a new tab by default

We don't ever want to take the user out of the official doc set. As soon as the user is taken out of the docs, we diminish the likelihood that they will make their way back, while also reducing their situational awareness of where they were in the doc set.

Requiring the writer to remember to add a caret^ in the AsciiDoc every time we want this behavior introduces a lot of potential for error, and thus, an inconsistent experience on the site. This inconsistency can create frustration if it occurs, and introduce anxiety over clicking links.

The writer should not be required to set the caret^ flag for every external link. Antora should handle this.