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 Python SDK PR adding ContainerBlock is merged and the dependency is updated.

Test plan

• Verify ContainerBlock import resolves after SDK update
• ruff check and ruff format --check pass
• pytest tests/blocks/test_container.py passes