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/prepare_elimination.html.twig

@@ -1,3 +1,4 @@
-<h6>{{ 'Prepare elimination'|trans({}, 'instructions') }}</h6>
-<p>{{ 'Choose a colour for each candidate: green means safe, red means eliminated.'|trans({}, 'instructions') }}</p>
-<p>{{ 'Use Save and start to play the elimination immediately, or save first and start later via the Results tab.'|trans({}, 'instructions') }}</p>
+{{ include([
+    'backoffice/help/' ~ app.request.locale ~ '/prepare_elimination.html.twig',
+    'backoffice/help/nl/prepare_elimination.html.twig',
+]) }}