[Feat] Server onboarding and guides

[greeeen-dev]

Checklist

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Problem

Some servers feel that it's difficult to learn how to use the essentials of Unifier, according to user feedback. This suggests a poor UX and it must be addressed.

Proposed solution

Add server onboarding which introduces the basic features of Unifier, and add "Next steps" to certain commands like /bridge create-room.

Alternatives considered

No response

Additional context

No response