docs: add Chapter 12 on logging (#6847)

**Staged:**
https://dart-dev--pr6847-feature-guide-fwe-chapter-12-r16tdtpx.web.app/get-started/logging

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Co-authored-by: Parker Lougheed <[email protected]>
Co-authored-by: Amanda Fitch <[email protected]>

Author: 3 people

src/content/get-started/index.md

@@ -36,3 +36,4 @@ follow the instructions on how to [Get the Dart SDK][].
 9. [Data and JSON](/get-started/data-and-json)
 10. [Testing](/get-started/testing)
 11. [Http](/get-started/http)
+12. [Logging](/get-started/logging)