docs(samples): Add Kafka to Dataflow snippet

[VeronicaWasson]

Description

Adds a snippet that shows how to write messages to Pub/Sub from Dataflow.

Bug: b/350789182
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]

Here is the summary of changes.

You are about to add 1 region tag.

• dataflow/snippets/read_kafka.py:16, tag dataflow_kafka_read

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[davidcavazos]

😮

[davidcavazos]

Looks like the linter is configured to ignore "self" type annotations, but not "cls".

[davidcavazos]

Updating linter rules on #12428