docs(samples): Read multiple Kafka topics from Dataflow

[VeronicaWasson]

Description

Add a code snippet that shows how to read from multiple Kafka topics in a Dataflow pipeline. This snippet shows a recommended pattern for handling messages from several Kafka topics within the same pipeline. This snippet will appear in a doc that outlines best practices for reading Kafka in Dataflow.

• Snippet bug: b/362324327
• Related doc bug: b/342015050

A Python version of this snippet is here: https://github.com/GoogleCloudPlatform/python-docs-samples/pull/12530

Checklist

• [ ] I have followed Sample Format Guide
• [ ] pom.xml parent set to latest shared-configuration
• [ ] Appropriate changes to README are included in PR
• [ ] 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)
• [ ] Tests pass: mvn clean verify required
• [ ] Lint passes: mvn -P lint checkstyle:check required
• [ ] Static Analysis: mvn -P lint clean compile pmd:cpd-check spotbugs:check advisory only
• [ ] 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 2 region tags.

• dataflow/snippets/src/main/java/com/example/dataflow/KafkaReadTopics.java:19, tag dataflow_kafka_read_multi_topic
• dataflow/snippets/src/main/java/com/example/dataflow/KafkaReadTopics.java:67, tag dataflow_kafka_read_multi_topic

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