Snippet `ConfigureAppConfiguration` in the generic host fundamental page was referencing an invalid variable name in the context of the scope. Updated to use the correct variable name.
Fixes#6760
The goals are ...
* Surface that env vars aren't added by default with the Generic Host.
* Describe `AddEnvironmentVariables`.
* Deal with env var prefix issue.
* Explain how to set env vars for dev (shoot reader to *Multiple environments* topic for more info). Mention VS, command-line, and VSC config.
* Add VSC example to the *Multiple environments* topic.
* Update the sample app and snippets for the topic.
Addresses #5495
* This one takes care of the topics in the *fundamentals/host* TOC area, including its three samples.
* All samples verified working with 2.1 RTM bits. 🎉
Ignore the three build warnings. They're fixed on https://github.com/aspnet/Docs/pull/6712.
Luke Latham
Mike McFarland
Miroslav Popovic
Daniel Moncada