docs: add v1 to v2 migration guide

Comprehensive migration guide covering:
- Every renamed/removed endpoint with before/after code examples
- Parameter mapping tables for all methods
- New FetchConfig/LlmConfig shared models
- Scheduled Jobs → Monitor namespace migration
- Crawl namespace changes (start/status/stop/resume)
- Removed features (mock mode, TOON, polling methods)
- Quick find-and-replace cheatsheet for fast migration
- Async client migration notes

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: VinciGit00