Add documentation on how to migrate from previous versions of `@cloudflare/vitest-pool-workers` to the newest minor version

### Existing documentation URL(s)

The update here (https://github.com/cloudflare/workers-sdk/pull/11632) to `@cloudflare/vitest-pool-workers` is a *breaking* change, despite being marked as a minor version upgrade. The changes in this package include a mandatory upgrade to Vitest v4, as well as new methods of defining a Cloudflare specific `vitest.config.ts`, changes to how mock outgoing fetches, and how to test using `SELF.fetch`.

We should absolutely have a migration guide for users, as we've hidden the "older" docs describing previous versions of the package, leaving users somewhat stranded without a clear guide on what needs to change.

https://3dbd7e73.preview.developers.cloudflare.com/workers/testing/vitest-integration/migration-guides/migrate-from-miniflare-2/ contains a previous versions of the docs.

### What changes are you suggesting?

**Requested change** Addition of a migration guide for the Vitest Pool Workers section, eg: https://3dbd7e73.preview.developers.cloudflare.com/workers/testing/vitest-integration/migration-guides/migrate-from-miniflare-2/. This should contain the changes necessary to migrate to versions of `@cloudflare/vitest-pool-workers@0.13.0` and above.

**Why?** This would help users navigate the breaking changes in the package, what was deprecated, and what changes they'll need to make.

**Outcome**: Users will have a guide to support upgrading between versions, as well as a reference to how the previous versions of the package worked.

### Additional information

_No response_