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/tab_candidates.html.twig

@@ -1,3 +1,5 @@
+<div class="row">
+<div class="col-xl-9 col-12">
 <div class="d-flex justify-content-between align-items-center mb-3">
     <div>
         {% set questions = quiz.questions %}
@@ -64,3 +66,8 @@
     </table>
     <button type="submit" class="btn btn-primary">{{ 'Save'|trans }}</button>
 </form>
+</div>
+<div class="col-xl-3 col-12">
+    {{ include('backoffice/help/quiz_answer_mapping.html.twig') }}
+</div>
+</div>