docs(samples): Add Dataflow snippet to read from multiple Kafka topics

[VeronicaWasson]

Description

Shows how to build a Dataflow pipeline that reads from multiple Kafka topics, filters based on the topic name, and performs different application logic per topic.

Snippet bug: b/362325397

Related doc bug: b/342015050

Checklist

• [ ] I have followed Sample Guidelines from AUTHORING_GUIDE.MD
• [ ] README is updated to include all relevant information
• [ ] Tests pass: nox -s py-3.9 (see Test Environment Setup)
• [ ] Lint pass: nox -s lint (see Test Environment Setup)
• [ ] These samples need a new API enabled in testing projects to pass (let us know which ones)
• [ ] These samples need a new/updated env vars in testing projects set to pass (let us know which ones)
• [ ] This sample adds a new sample directory, and I updated the CODEOWNERS file with the codeowners for this sample
• [ ] This sample adds a new Product API, and I updated the Blunderbuss issue/PR auto-assigner with the codeowners for this sample
• [ ] Please merge this PR for me once it is approved

[snippet-bot[bot]]

Here is the summary of changes.

You are about to add 1 region tag.

• dataflow/snippets/read_kafka_multi_topic.py:16, tag dataflow_kafka_read_multi_topic

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[VeronicaWasson]

LGTM! But there seems to be a merge conflict that needs resolving.

thanks!