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)