Add Azure install docs for Sourcegraph Server

Created by: sqs

@ctolkien requested Azure install docs for Sourcegraph Serve at https://twitter.com/c_tolkien/status/1009689484361809921. We currently only have AWS, GCP, and DigitalOcean docs for Sourcegraph Server, although we do have Azure docs for Sourcegraph Data Center (https://github.com/sourcegraph/deploy-sourcegraph/blob/master/docs/k8s.azure.md).

We should add Azure install docs for Sourcegraph Server.