samples(documentai, discoveryengine): Add Samples for Vertex AI Search Standalone RAG APIs

[holtskinner]

• https://cloud.google.com/generative-ai-app-builder/docs/builder-apis

[snippet-bot]

Here is the summary of changes.

You are about to add 5 region tags.

• discoveryengine/standalone_apis_sample.py:22, tag genappbuilder_check_grounding
• discoveryengine/standalone_apis_sample.py:80, tag genappbuilder_rank
• documentai/snippets/handle_response_sample_v1beta3.py:36, tag documentai_process_layout_document
• documentai/snippets/handle_response_sample_v1beta3.py:77, tag documentai_process_summarizer_document
• documentai/snippets/handle_response_sample_v1beta3.py:221, tag documentai_process_layout_document

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[dandhlee]

Reviewing for Sample styles.