Docs: Add documentation for the new global.json options in 3.0

[nguerrera]

See https://github.com/dotnet/core-setup/pull/6953 and https://github.com/dotnet/designs/pull/71

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Document Details

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• ID: a94c335b-0b65-96cc-1c25-65777fbe4bb5
• Version Independent ID: 6d80613f-42dc-35d7-aeff-3187a36dbbca
• Content: global.json overview - .NET Core CLI
• Content Source: docs/core/tools/global-json.md
• Product: dotnet-core
• Technology: dotnet-cli
• GitHub Login: @mairaw
• Microsoft Alias: mairaw

[mairaw]

Thanks for filing the issues for some of the CLI features @nguerrera. It's been in my backlog to do that. I'm adding this to our next sprint.