Update docs for webhooks

[lawreka]

Merge only after https://github.com/mintlify/mint/pull/2349

[hahnbeelee]

wait it might be worth adding a webhooks section to this page
https://mintlify-kathryn-webhooks-update.mintlify.app/api-playground/openapi/advanced-features

w. like an example of what it looks like like kinda like:

<CodeGroup>
  ```md Example
  ---
  title: "Get users"
  openapi: "/path/to/openapi-1.json WEBHOOK /users"
  ---
  ```

  ```md Format
  ---
  title: "title of the page"
  openapi: openapi-file-path WEBHOOK path
  ---
  ```
</CodeGroup>

[hahnbeelee]

one comment but otherwise lgtm!~