From near the top of the page:

Webhooks currently notify you about changes to pages and databases — such as when a new page is created, a title is updated, or someone changes a database schema. The events themselves do not contain the full content that changed. Instead, the webhook acts as a signal that something changed, and it’s up to your integration to follow up with a call to the Notion API to retrieve the latest content.

The body of the webhook request will not contain any specific details about the page created, it will just list relevant information related to the creation event. Who did it, when it happened, etc. If you want more details like title or custom properties of the page created, you should use the ID provided by the webhook payload to look up that information separately.