[Workers] Add docs for Vitest testing

[admah]

Adds new Testing section to the Workers docs that contains:

• setup and install instructions for @cloudflare/vitest-pool-workers
• API docs for @cloudflare/vitest-pool-workers
• Recipes to help users with a variety of test scenarios
• Migration guides for unstable_dev and Miniflare 2's Vitest environment

It also moves the local dev guide from Observability into this section.

[cloudflare-workers-and-pages[bot]]

Deploying with    Cloudflare Pages

Latest commit:	7aae21d
Status:	✅  Deploy successful!
Preview URL:	https://31c4174e.cloudflare-docs-7ou.pages.dev
Branch Preview URL:	https://admah-vitest-pool-docs.cloudflare-docs-7ou.pages.dev

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[github-actions[bot]]

This PR changes current filenames or deletes current files. Make sure you have redirects set up to cover the following paths:

• [ ] /workers/observability/local-development-and-testing/

[github-actions[bot]]

Files with changes (up to 15)

Original Link	Updated Link
https://developers.cloudflare.com/workers/testing/vitest-integration/test-apis/	https://admah-vitest-pool-docs.cloudflare-docs-7ou.pages.dev/workers/testing/vitest-integration/test-apis/
https://developers.cloudflare.com/workers/testing/vitest-integration/get-started/write-your-first-test/	https://admah-vitest-pool-docs.cloudflare-docs-7ou.pages.dev/workers/testing/vitest-integration/get-started/write-your-first-test/
https://developers.cloudflare.com/workers/testing/vitest-integration/get-started/migrate-from-miniflare-2/	https://admah-vitest-pool-docs.cloudflare-docs-7ou.pages.dev/workers/testing/vitest-integration/get-started/migrate-from-miniflare-2/
https://developers.cloudflare.com/workers/testing/vitest-integration/configuration/	https://admah-vitest-pool-docs.cloudflare-docs-7ou.pages.dev/workers/testing/vitest-integration/configuration/
https://developers.cloudflare.com/workers/testing/integration-testing/	https://admah-vitest-pool-docs.cloudflare-docs-7ou.pages.dev/workers/testing/integration-testing/
https://developers.cloudflare.com/workers/observability/local-development-and-testing/	https://admah-vitest-pool-docs.cloudflare-docs-7ou.pages.dev/workers/observability/local-development-and-testing/
https://developers.cloudflare.com/workers/testing/vitest-integration/get-started/migrate-from-unstable-dev/	https://admah-vitest-pool-docs.cloudflare-docs-7ou.pages.dev/workers/testing/vitest-integration/get-started/migrate-from-unstable-dev/
https://developers.cloudflare.com/workers/testing/local-development/	https://admah-vitest-pool-docs.cloudflare-docs-7ou.pages.dev/workers/testing/local-development/
https://developers.cloudflare.com/workers/testing/debugging-tools/	https://admah-vitest-pool-docs.cloudflare-docs-7ou.pages.dev/workers/testing/debugging-tools/
https://developers.cloudflare.com/workers/testing/vitest-integration/isolation-and-concurrency/	https://admah-vitest-pool-docs.cloudflare-docs-7ou.pages.dev/workers/testing/vitest-integration/isolation-and-concurrency/
https://developers.cloudflare.com/workers/testing/vitest-integration/get-started/	https://admah-vitest-pool-docs.cloudflare-docs-7ou.pages.dev/workers/testing/vitest-integration/get-started/
https://developers.cloudflare.com/workers/testing/unit-testing/	https://admah-vitest-pool-docs.cloudflare-docs-7ou.pages.dev/workers/testing/unit-testing/
https://developers.cloudflare.com/workers/testing/	https://admah-vitest-pool-docs.cloudflare-docs-7ou.pages.dev/workers/testing/
https://developers.cloudflare.com/workers/testing/vitest-integration/recipes/	https://admah-vitest-pool-docs.cloudflare-docs-7ou.pages.dev/workers/testing/vitest-integration/recipes/
https://developers.cloudflare.com/workers/testing/vitest-integration/known-issues/	https://admah-vitest-pool-docs.cloudflare-docs-7ou.pages.dev/workers/testing/vitest-integration/known-issues/

[admah]

@deadlypants1973 this is ready for review.