Zone page: links to the same zone in the other views

The header says "also in VIEW" for every zone of the same name in
another view, linked or not, in the view's colour, keeping the open tab
where the other zone has it. The zone API gains in_views.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/views.md

@@ -168,6 +168,10 @@ shows one (it needs `tsig:write` and is audited). Key names starting with
 `jarl-view-` are reserved, and the view keys can be used in your own
 options (`allow-transfer { key jarl-view-external; }`).
 
+**Moving between views.** A zone's page lists the other views that have a
+zone of the same name ("also in external", in each view's colour). Each
+one links straight to that zone, on the tab you are on.
+
 ## Linked zones
 
 Most split-horizon zones are the same in every view apart from a few