System.CommandLine docs update

[adamsitnik]

High level overview:

1. I've merged the doc that explains the syntax (https://learn.microsoft.com/en-us/dotnet/standard/commandline/syntax) without showing the code and the doc that shows the related code (https://learn.microsoft.com/en-us/dotnet/standard/commandline/define-commands) into one.
2. I've moved the "Command-line design guidance" from syntax doc to a dedicated doc (it's big and important, but the very new users don't need to ready it when learning about syntax for the first time).
3. I've introduced a dedicated doc about how to parse and invoke to show it's two separate things.
4. I've introduced dedicated doc about other features such as parsing and validation etc
5. I've simplified the tutorial to start with a code that does not use invoking at all, just to show all its benefits and gradually use more features.

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Internal previews

Toggle expand/collapse

📄 File	🔗 Preview link
docs/fundamentals/toc.yml	docs/fundamentals/toc
docs/standard/commandline/design-guidance.md	Design guidance
docs/standard/commandline/get-started-tutorial.md	Tutorial: Get started with System.CommandLine
docs/standard/commandline/how-to-configure-the-parser.md	How to configure the parser in System.CommandLine
docs/standard/commandline/how-to-customize-help.md	Customize help output
docs/standard/commandline/how-to-customize-parsing-and-validation.md	docs/standard/commandline/how-to-customize-parsing-and-validation
docs/standard/commandline/how-to-enable-tab-completion.md	Tab completion for System.CommandLine
docs/standard/commandline/how-to-parse-and-invoke.md	"How to parse and invoke the result"
docs/standard/commandline/index.md	System.CommandLine overview
docs/standard/commandline/migration-guide-2.0.0-beta5.md	docs/standard/commandline/migration-guide-2.0.0-beta5
docs/standard/commandline/syntax.md	Syntax overview: Commands, options, and arguments

[gewarren]

Thanks for adding all these docs!

[adegeo]

It's really awesome to see some traction in this area, thanks you! Here are some improvements I found.