Migration guide from APIOps Toolkit to APIOps CLI

## Summary

Create documentation (and/or a guided walkthrough) that helps customers migrate from the existing [APIOps Toolkit](https://github.com/Azure/apiops) to APIOps CLI.

## Problem

Customers currently using APIOps Toolkit need a clear path to adopt APIOps CLI. Without a migration guide, they face uncertainty about:
- How their existing extracted artifacts map to the new CLI's format
- How to recreate their CI/CD pipelines
- How to migrate their configuration (filters, overrides)
- What new capabilities they gain (e.g., Copilot-assisted configuration setup per #120)

## Proposed Content

### 1. Conceptual Mapping
- Side-by-side comparison of Toolkit vs CLI concepts (extractor → `apiops extract`, publisher → `apiops publish`, etc.)
- Differences in artifact format and directory structure

### 2. Step-by-Step Migration
- How to initialize a new repo with `apiops init`
- How to migrate existing extracted artifacts (or re-extract from scratch)
- How to convert Toolkit configuration files to CLI configuration format
- How to update CI/CD pipelines (GitHub Actions / Azure DevOps)

### 3. Leveraging New Features
- Using the interactive Copilot-assisted configuration prompt (#120) to generate new configuration files from migrated artifacts
- New capabilities not available in Toolkit (incremental publish, etc.)

### 4. FAQ / Troubleshooting
- Common migration pitfalls
- Breaking changes between Toolkit and CLI

## Related

- #120 — Interactive configuration prompt (relevant to migration step where users set up new config files)

Verwenden Sie die vorgeschlagenen Inhaltsabschnitte als anfängliche Checkliste. Beginnen Sie mit dem Konzept und der Artefaktzuordnung von Toolkit-to-CLI und überprüfen Sie anschließend die Konfiguration sowie die CI/CD- und Migrationsschritte. Nehmen Sie den von Copilot unterstützten Konfigurations-Prompt aus #120, neue Funktionen und die Fehlerbehebung auf; abgeschlossen bedeutet, dass der Leitfaden Kunden einen vollständigen, umsetzbaren Migrationspfad bietet.

Vom Indexierungsmodell aus dem Issue-Text verfasst.