refactor: move help content out of translations into locale-specific partials

Replace the instructions translation domain with plain HTML files under
templates/backoffice/help/nl/. Each help/*.html.twig is now a locale
dispatch shim that tries the current locale first and falls back to nl,
so adding English (or any other language) is simply a matter of creating
a help/en/ directory with the translated files — no code changes needed.

Removes instructions+intl-icu.nl.xliff.

templates/backoffice/help/quiz_add_blank.html.twig

@@ -1,3 +1,4 @@
-<h6>{{ 'Create blank quiz'|trans({}, 'instructions') }}</h6>
-<p>{{ 'Create an empty quiz and add questions from the question bank. Useful if you reuse questions or have prepared them in the bank in advance.'|trans({}, 'instructions') }}</p>
-<p>{{ 'After creating, open the quiz, add questions via the Overview tab and finalize the quiz before activating it.'|trans({}, 'instructions') }}</p>
+{{ include([
+    'backoffice/help/' ~ app.request.locale ~ '/quiz_add_blank.html.twig',
+    'backoffice/help/nl/quiz_add_blank.html.twig',
+]) }}