Add maintenance window docs

[Kemi-Elizabeth]

Priority

3 - Not urgent - can be addressed as team bandwidth permits

Where should the changes be made?

Add maintenance window docs to https://docs.upsun.com/learn/overview/build-deploy.html#zero-downtime-deployments

What exactly should be updated?

https://docs.upsun.com/learn/overview/build-deploy.html#zero-downtime-deployments

Optional: Additional context

No response