feat: add migration guide for moderation (CHA-2591)

Covers: adding/removing moderators, banning/unbanning (app-level and
channel-level), shadow banning, querying banned users, muting/unmuting
users. Each section shows Before/After code blocks with key differences.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: mogita