ML.NET CLI P0 Changes based on User Study

[LittleLittleCloud]

doc

Work items:

• Make required commands more obvious
• • Add Require flag to option invocation hint text. (it's a bonus of using System.Commandline.Api, with this we don't need to add "required" flag in helper text)
• • Add Required option section to the end of each of help text.
• Aka help links in each --help which links to more info + datasets. (when will helper link ready?)
• Put commands / options in alphabetical order.
• Make recommend > recommendation for consistency. (This is P1 but it's a small change)
• Generated projects should be .NET Core 3.1