feat: (Gen AI) Update the `document_retrieval` sample for embeddings

[Valeriy-Burlaka]

Description

Updates the sample for the "Get text embeddings for a snippet of text" section of the embeddings page:

• Fix the Example response
• Provide more clear and idiomatic example of using the Vertex SDK
• Show alternative ways to provide the input for the get_embeddings method.

Checklist

• [x] I have followed Sample Guidelines from AUTHORING_GUIDE.MD
• [ ] README is updated to include all relevant information
• [x] Tests pass: nox -s py-3.8 && nox -s py-3.11
• [x] Lint pass: nox -s lint
• [ ] 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
• [x] Please merge this PR for me once it is approved

[Valeriy-Burlaka]

@msampathkumar, following the review comments to this PR, I can't decide if you deem these fixes helpful or not. If you believe that everything is OK with the sample in its current state, I can simply close this PR. Thanks!

[msampathkumar]

Hi @Valeriy-Burlaka Thank you for your work on this. For now, marking this request as closed.