[docs] Add consumer migration guide (#72)

[coisa]

Summary

• Add a step-by-step migration guide for consumer repositories adopting Fast Forward DevTools.
• Document expected sync changes, review focus areas, and branch protection considerations.
• Link the guide from the existing sync command documentation and usage navigation.

Verification

• git diff --check
• composer dev-tools docs

Closes #72

[github-actions]

🚀 Preview is available for this pull request.

• Docs: https://php-fast-forward.github.io/dev-tools/previews/pr-78/
• Coverage: https://php-fast-forward.github.io/dev-tools/previews/pr-78/coverage/