Document overriding app settings in Azure Portal

[scottaddie]

https://docs.microsoft.com/en-us/aspnet/core/fundamentals/configuration/

This document doesn't talk about how the Application settings blade in the Azure Portal can be used to override app settings. I customer asked about it this morning. It's a common task that we should document. I suggest a new section in the document linked above.

[guardrex]

@scottaddie Would you like me to take this one on for the updates on #7102?

[scottaddie]

Yes, please! I've had a couple customers ask about this now.