docs: add code snippets for llm text generation

[ashleyxuu]

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Fixes internal #338873779 🦕

[snippet-bot]

Here is the summary of changes.

You are about to add 3 region tags.

• samples/snippets/text_generation_test.py:24, tag bigquery_dataframes_generate_text_tutorial_create_remote_model
• samples/snippets/text_generation_test.py:35, tag bigquery_dataframes_generate_text_tutorial_perform_keyword_extraction
• samples/snippets/text_generation_test.py:52, tag bigquery_dataframes_generate_text_tutorial_perform_sentiment_analysis

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[tswast]

Thanks! A few nitpicks.

[tswast]

Thanks!

[tswast]

I've mailed internal CL 632203837 to address the permissions error in the remote functions sample.