Migration guide update

[ethanpalm]

Documentation changes

This PR updates https://mintlify.com/docs/guides/migration to include:

• More information about the scraper (where files are stored, where to put them after you scrape)
• High level guidance for doing a manual migration
• Decision matrix for if you can use the scraper or not
• Include GitBook as beta for automated migrations since it works, but not perfectly

Closes https://linear.app/mintlify/issue/DOC-22/migrations-guide-update

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