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/season_settings.html.twig

@@ -1,3 +1,4 @@
-<h6>{{ 'Season settings'|trans({}, 'instructions') }}</h6>
-<p>{{ 'Adjust the name and other settings for this season here.'|trans({}, 'instructions') }}</p>
-<p>{{ 'The season code is fixed after creation and determines the URL candidates use to log in.'|trans({}, 'instructions') }}</p>
+{{ include([
+    'backoffice/help/' ~ app.request.locale ~ '/season_settings.html.twig',
+    'backoffice/help/nl/season_settings.html.twig',
+]) }}