[DE] Update `Client/Beatmap_editor/Menu`

[The-Last-Cookie]

This article needed a rewrite because of grammar issues, some untranslated phrases and also the wrong pronouns (formal Sie instead of informal du which was chosen for the wiki)

• [ ] Remove needs_native_review if PR gets reviewed
• [ ] Check if Beatmap-Thread should be renamed to Beatmap-Diskussion

Self-check

• [x] The changes are tested against the contribution checklist
• [ ] (translations only) The changes are reviewed on GitHub by a fluent speaker