Testing documentation

[csc530]

I think there should be dedicated documentation for using the Spectre.Console.Testing classes on the main website as it's a little hidden that SpectreConsole supports testing with it. And it's not super easy looking through the source code test to figure out which classes to use and how for whatever purpose you have (as seen in #912).
Right now there is a blurb under best practices...

Unit Testing Best Practices
For testing of console output, Spectre.Console has IAnsiConsole that can be injected into your application. The Spectre.Console.Test contains a set of utilities for capturing the output for verification, either manually or via a tool such as Verify.

I think it could be featured more prominently in its own section with use cases, examples, etc. (I'm also happy to help start/contribute to these docs too).

[patriksvensson]

@csc530 Sounds like a very good idea. PR:s for this are more than welcome! 😄