[DOCS-14494] Use region-param shortcode for webhook URL

[buraizu]

What does this PR do?

Replaces the hardcoded https://app.datadoghq.com/intake/webhook/zabbix host in the Zabbix integration README's webhook script with the {{< region-param key="dd_full_site" >}} shortcode, so the webhook URL resolves to the reader's Datadog site instead of always showing US1.

Motivation

Customers not on the US1 site otherwise have to know that the host needs to change. This matches the pattern already used in the github and azure_devops READMEs and the Amazon SNS update (DataDog/integrations-internal-core#3306).

Fixes DOCS-14494

Review checklist

• PR has a meaningful title or PR has the no-changelog label attached
• Feature or bugfix has tests
• Git history is clean
• If PR impacts documentation, docs team has been notified or an issue has been opened on the documentation repo

Additional Notes

Documentation-only change (integration README). No code or changelog impact.