Fix Keycloak DevUI template to use a configured dev root

[sberyozkin]

Fixes #20812

[sberyozkin]

@phillip-kruger Thanks Philipp, I forgot update the doc as it refers to q/dev, I'll update once I do it

[sberyozkin]

@phillip-kruger I've added a note about replacing q if customizing quarkus.http.root-path and/or quarkus.http.non-application-root-path - q/dev is referred to in at least 5 places in the doc so I thought adding a small section, similarly to how it is done in the Dev UI doc, would be more useful than trying to update every occurrence with the note; I'll merge unless you'll suggest some text updates, would not be a problem at all