Replace all em-dashes with semicolons or colons throughout the nl help
partials. Also replace remaining 'XLSX-bestand' with 'Excel-bestand' in
season_tests and index, and also also fix the em-dash that was still on
the same line as the 'speelronde -> spel' fix in index.
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.
Marijn
