Testing documentation

[patriksvensson]

Discussed in #1409

^{Originally posted by csc530 December 22, 2023}
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).

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[csc530]

I made a PR that adds some pages to show the existence of the testing classes and how to use them.
Although I'm not sure how to add API documentation for the test classes to docs.
I tried adding the line "../../src/Spectre.Console.Testing/**/{!bin,!obj,!packages,!*.Tests,}/**/*.cs", to the sourceFiles setting list in the docs/Program.cs file, but it didn't seem to work.