docs: document existing-site imports

[yoldaolmak]

Summary

• add a guided existing-site import page based on the current workflow
• document discovery, grouping, model review, preview, and import steps
• state the current limitations around JavaScript rendering, remote images, public-only pages, and retry duplicates
• link the guide from the documentation home page

Validation

• pnpm --filter docs validate
• pnpm --filter docs tc
• pnpm --filter docs build
• pnpm exec vp run ready

Closes #38

[coderabbitai]

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