Add webhook payload schema reference for notifications

[rianvdm]

Summary

Documents the generic webhook payload schema for Cloudflare notifications, a frequently asked question from customers.

What this PR adds

New page: /notifications/reference/webhook-payload-schema/

Covers:

• Payload structure — All fields in the generic webhook JSON payload
• Field descriptions — Table explaining each field's type and purpose
• Example payloads — Seven collapsible examples for common alert types (DDoS L4/L7, SSL certificate, health check, Workers, Access certificate, Workers observability)
• Webhook validation — How to use cf-webhook-auth header

Updated: /notifications/get-started/configure-webhooks.mdx

• Added link to new schema reference page from the generic webhooks section

Why

Questions about the format of webhook alert payloads come up frequently. This information was previously only available on an internal wiki page.

[github-actions]

This pull request requires reviews from CODEOWNERS as it changes files that match the following patterns:

Pattern	Owners
/src/content/docs/notifications/	@dcpena, @cloudflare/pcx-technical-writing

[github-actions]

Preview URL: https://64d4632a.preview.developers.cloudflare.com
Preview Branch URL: https://rian-alert-webhook-payload-v2.preview.developers.cloudflare.com

Files with changes (up to 15)

Original Link	Updated Link
https://developers.cloudflare.com/notifications/reference/webhook-payload-schema/	https://rian-alert-webhook-payload-v2.preview.developers.cloudflare.com/notifications/reference/webhook-payload-schema/
https://developers.cloudflare.com/notifications/get-started/configure-webhooks/	https://rian-alert-webhook-payload-v2.preview.developers.cloudflare.com/notifications/get-started/configure-webhooks/

[rianvdm]

cc @jasoncabot for review