Move to the migration guide at Buf docs

[jrinehart-buf]

This PR should not be merged until Buf docs are updated to contain the protoc-gen-validate migration guide.

This PR:

1. Changes internal links to the migration guide to point to the new URL that lives under buf.build/docs
2. Drops the migration guide from this repository.

This PR does not:

1. Fix the invalid link in the migration guide referring to legacy mode: that's part of the new guide at buf.build/docs.

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The latest Buf updates on your PR. Results from workflow Buf CI / buf (pull_request).

Build	Format	Lint	Breaking	Updated (UTC)
✅ passed	✅ passed	✅ passed	✅ passed	Jan 15, 2025, 7:20 PM