docs: update CONTRIBUTING.md with documentation about container structure tests

[lukasholzer]

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Summary

Document the new testing with Container Structure Tests

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For us to review and ship your PR efficiently, please perform the following steps:

• [x] Open a bug/issue before writing your code 🧑‍💻. This ensures we can discuss the changes and get feedback from everyone that should be involved. If you`re fixing a typo or something that`s on fire 🔥 (e.g. incident related), you can skip this step.
• [x] Read the contribution guidelines 📖. This ensures your code follows our style guide and passes our tests.
• [x] Update or add tests (if any source code was changed or added) 🧪
• [x] Update the included software doc (if you updated included software) 📄
• [x] Update or add documentation (if features were changed or added) 📝
• [x] Make sure the status checks below are successful ✅

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