docs: add documentation for Zanzibar dashboard

I just realized no documentation for Zanzibar dashboard

<img width="731" alt="Image" src="https://github.com/user-attachments/assets/90d50211-97cd-4727-878b-405dee91a738" />

- URL for Zanzibar dashboard

https://geohub.data.undp.org/dashboards/zanzibar

- URL for dashboard documentation

https://docs.undpgeohub.org/dashboards/