samples(discoveryengine): Add AlloyDB Import Sample

[holtskinner]

To be added to https://cloud.google.com/generative-ai-app-builder/docs/create-data-store-es

[snippet-bot]

Here is the summary of changes.

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• discoveryengine/import_documents_sample.py:468, tag genappbuilder_import_documents_alloy_db

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[dandhlee]

Reviewing for Python samples style.