Add container registry cleanup policy example in the docs

🤔 What does this MR do?

How cleanup policy parameters/rules are applied is a source of confusion.

This MR adds an example for https://docs.gitlab.com/ee/user/packages/container_registry/reduce_container_registry_storage.html#how-the-cleanup-policy-works.

Among other things, it's important to show that rules that preserve tags have precedence over rules that expire tags.

⛓ Related issues

Example of confusion around cleanup policies rules: #371467 (closed)

🏎 Author's checklist

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☑ Reviewer's checklist

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