feat: add container block example

[srtaalej]

Summary

• Adds a concise ContainerBlock example matching the docs
• A collapsible container with sections, dividers, context, and actions (bulk update scenario)
• Includes implementation, test, and README entry

Note: Tests will pass once the Java SDK PR adding ContainerBlock is merged and the dependency version is bumped.

Test plan

• Verify ContainerBlock class resolves after SDK update
• mvn spotless:check passes
• mvn test passes

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