Move to the migration guide at Buf docs (#294)

**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.

README.md

@@ -252,7 +252,7 @@ Offered under the [Apache 2 license][license].
 [file-bug]: https://github.com/bufbuild/protovalidate/issues/new?assignees=&labels=Bug&template=bug_report.md&title=%5BBUG%5D
 [file-feature-request]: https://github.com/bufbuild/protovalidate/issues/new?assignees=&labels=Feature&template=feature_request.md&title=%5BFeature+Request%5D
 [license]: https://github.com/bufbuild/protovalidate/blob/main/LICENSE
-[migrate]: https://github.com/bufbuild/protovalidate/blob/main/docs/migrate.md
+[migrate]: https://buf.build/docs/migration-guides/migrate-from-protoc-gen-validate/
 [migration-tool]: https://github.com/bufbuild/protovalidate/blob/main/tools/protovalidate-migrate
 [pgv]: https://github.com/bufbuild/protoc-gen-validate
 [pv-go]: https://github.com/bufbuild/protovalidate-go

docs/README.md

@@ -47,7 +47,9 @@ existing projects to `protovalidate`.
   Conformance tool. Learn how to use this tool to ensure all implementations
   align with `protovalidate`'s rules and constraints effectively.
 
-- [Migrate](migrate.md): If you're planning to migrate your existing project to
+- [Migrate][migrate]: If you're planning to migrate your existing project to
   `protovalidate`, this guide is for you. The Migrate tool is designed to help
   you transition smoothly, minimizing any potential disruption to your project's
   development.
+
+[migrate]: https://buf.build/docs/migration-guides/migrate-from-protoc-gen-validate/

tools/protovalidate-migrate/README.md

@@ -0,0 +1,10 @@
+![The Buf logo](https://raw.githubusercontent.com/bufbuild/protovalidate/main/.github/buf-logo.svg)
+
+# Migrating from `protoc-gen-validate`
+
+Migrating from `protoc-gen-validate` to `protovalidate` should be safe, incremental, and relatively painless. 
+
+Visit [Buf's documentation][migrate] to learn about automated and manual migrations.
+
+[migrate]: https://buf.build/docs/migration-guides/migrate-from-protoc-gen-validate/
+