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/help/season_tests.html.twig

@@ -0,0 +1,11 @@
+<h6>{{ 'Managing quizzes'|trans({}, 'instructions') }}</h6>
+<p>{{ 'Add a quiz from an XLSX file or create a blank one and fill it via the question bank. Then open the quiz to review and finalize it.'|trans({}, 'instructions') }}</p>
+<p>{{ 'A quiz must be finalized before it can be activated. Only one quiz can be active at a time — this is the quiz candidates can currently take.'|trans({}, 'instructions') }}</p>
+<h6>{{ 'Order of operations'|trans({}, 'instructions') }}</h6>
+<ol>
+    <li>{{ 'Create the quiz (XLSX or blank)'|trans({}, 'instructions') }}</li>
+    <li>{{ 'Review questions and finalize the quiz'|trans({}, 'instructions') }}</li>
+    <li>{{ 'Activate the quiz'|trans({}, 'instructions') }}</li>
+    <li>{{ 'Let candidates take the quiz'|trans({}, 'instructions') }}</li>
+    <li>{{ 'View results and prepare the elimination'|trans({}, 'instructions') }}</li>
+</ol>