docs(samples): Added Vertex AI Search "lite" method sample

[holtskinner]

https://cloud.google.com/generative-ai-app-builder/docs/preview-search-results#web-api-key

Description

Fixes #

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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.

• discoveryengine/search_lite_sample.py:16, tag genappbuilder_search_lite

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[holtskinner]

Tests require #12777 to include the new secret

[msampathkumar]

LGTM.