Updates to appcat migration docs from GitHub

[alexwolfmsft]

Summary

This PR takes the additional documentation from GitHub for the AppCat tool and adds it to the docs.

Fixes #Issue_Number (if available)

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Internal previews

📄 File	🔗 Preview link
docs/azure/migration/appcat/custom-configuration.md	docs/azure/migration/appcat/custom-configuration
docs/azure/migration/appcat/faq.md	docs/azure/migration/appcat/faq