[How-to Guide]: Use the Microsoft Graph Developer Proxy with national cloud deployments

[garrytrinder]

Microsoft Graph Developer Proxy supports the use of national cloud deployments, however we don't call out what steps developers need to take to take advantage of this in our guides, we should add a How-to guide to cover this.

[waldekmastykarz]

Good catch! Seems like we totally missed that 😊

[waldekmastykarz]

Following this spec: #70 we'll be removing this functionality in favor of URLs that are controlled by users. Because of this, we no longer need this documentation.