feat: add contextual help panels to all backoffice pages

Add a 50/50 or 66/33 split layout to every backoffice page with Dutch
instructions explaining how to use Tijd voor de Test. Content covers the
overall workflow, quiz finalize/activate flow, and both candidate
participation methods (own device vs. shared laptop).

Help text lives in dedicated partials under templates/backoffice/help/ and
is loaded via Twig include(), keeping page templates clean. All strings use
a separate 'instructions' translation domain (instructions+intl-icu.nl.xliff)
isolated from the main messages domain.

Also updates 'Finalize'-related Dutch translations to use 'Afronden' and
adds tooltips to the finalize/unfinalize buttons.

templates/backoffice/quiz_add_blank.html.twig

@@ -20,7 +20,7 @@
             {{ form_end(form) }}
         </div>
         <div class="col-md-6 col-12">
-            <p class="mb-3">{{ 'Create an empty quiz and add questions from the question bank.'|trans }}</p>
+            {{ include('backoffice/help/quiz_add_blank.html.twig') }}
         </div>
     </div>
 {% endblock %}