docs: add info about testing custom formatters

[davidjgoss]

🤔 What's changed?

Add documentation about testing custom formatters, and link to the pretty formatter as a good example.

⚡️ What's your motivation?

Fixes #1545.

🏷️ What kind of change is this?

• 🏦 Refactoring/debt/DX (improvement to code design, tooling, documentation etc. without changing behaviour)

📋 Checklist:

• I agree to respect and uphold the Cucumber Community Code of Conduct
• I've changed the behaviour of the code
  • I have added/updated tests to cover my changes.
• My change requires a change to the documentation.
  • I have updated the documentation accordingly.
• Users should know about my change
  • I have added an entry to the "Unreleased" section of the CHANGELOG, linking to this pull request.

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[coveralls]

Coverage remained the same at 98.299% when pulling 4e0219a on docs-custom-formatter-testing into 4dac0a7 on main.