docs: clarify generated image build test docs

[iacker]

Summary

• make the generated integration test docs for validateImageCommands describe the actual image-build path in more detail
• mention the Dockerfile fixture, copied content.txt, on-demand buildkitd startup, and runtime image verification
• regenerate site/content/en/docs/contrib/tests.en.md

Testing

• PATH="/opt/homebrew/bin:$PATH" go test ./cmd/minikube/cmd -run TestGenerateTestDocs
• PATH="/opt/homebrew/bin:$PATH" make generate-docs

Closes #11571

[k8s-ci-robot]

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[k8s-ci-robot]

Hi @iacker. Thanks for your PR.

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[minikube-bot]

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