Add information about webhook payload types

[Migaroez]

📋 Description

Adds information to the webhooks article about the new and improved payload types including

• What has changed.
• How long the old things keep existing.
• How to change things before it gets changed for you.

✅ Contributor Checklist

I've followed the Umbraco Documentation Style Guide and can confirm that:

• Code blocks are correctly formatted.
• Sentences are short and clear (preferably under 25 words).
• Passive voice and first-person language (“we”, “I”) are avoided.
• Relevant pages are linked.
• All links work and point to the correct resources.
• Screenshots or diagrams are included if useful.
• Any code examples or instructions have been tested.
• Typos, broken links, and broken images are fixed.

Product & Version (if relevant)

Cms V16

Deadline (if relevant)

None, already shipped.

[eshanrnh]

Thanks for the PR, @Migaroez 🙌 The PR looks great 💪