[DE] Add `osu!tourney/Multiplayer_usage`, `osu!tourney/Setup` and `osu!tourney/Skinning`

[The-Last-Cookie]

This is part of a series that aims to add all articles under osu!_tournament_client.

Self-check

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