docs: Deprecate references to `@octokit/webhooks-definitions`

[rapsealk]

Hello actions/toolkit!

I've noticed that the document still references @octokit/webhooks-definitions, which has been deprecated by https://github.com/octokit/webhooks/issues/447.

This PR updates the reference from @octokit/webhooks-definitions to @octokit/webhooks-types to avoid confusion.

[rapsealk]

@bdehamer Hi! May I kindly request your review for these changes? Thank you!

[jmendiara]

Up! Having this PR merged would saved me some time researching why some events are not defined in the recommended @octokit/webhooks-definitions

[wolfy1339]

The @octokit/webhooks-types package will be deprecated in the near future. It's best to upgrade your @octokit/webhooks version and use @octokit/openapi-webhooks-types instead.

Instead of community definitions, we switched to GitHub's official OpenAPI definitions