Add testing documentation

[FrankRay78]

The following PR spectreconsole/examples#2 should be reviewed/merged at the same time as this one.

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fixes #1410

Addresses:

• Struggling with testing a console app #912
• What is the correct way to handle user input to IAnsiConsole when implementing unit tests? #1524

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• I have read the Contribution Guidelines
• I have commented on the issue above and discussed the intended changes
• A maintainer has signed off on the changes and the issue was assigned to me
• All newly added code is adequately covered by tests
• All existing tests are still running without errors
• The documentation was modified to reflect the changes OR no documentation changes are required.

Changes

• Wrote an entirely new documentation page for Unit Testing
• Updated site index and best practices with unit testing content
• Added Spectre.Console.Testing namespace to generated API docs
• Minor grammar improvements

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Please upvote 👍 this pull request if you are interested in it.

[FrankRay78]

This one's ready for review.

IMPORTANT: The following PR spectreconsole/examples#2 should be reviewed/merged at the same time as this one.