feat: add disaster recovery guidance for NGINXaaS for Azure deployments (#493)

* add disaster recovery guidance for NGINXaaS for Azure deployments

* address more comments

* address all comments, final draft

* add comment about configuring identical upstreams in the secondary region

* Update content/nginxaas-azure/disaster-recovery.md

* Apply suggestions from code review

formatting changes

* address comments re: how to execute all the code

* docs: convert notes to shortcode

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Co-authored-by: Jon Torre <[email protected]>
Co-authored-by: Jon Cahill-Torre <[email protected]>

Author: kuthiala