chore: add a notebook about using AI functions for movie posters (googleapis#2537)

This will be the reference notebook to be used by the tech blog on AI
functions in BigFrames

Author: sycai

docs/user_guide/index.rst

@@ -44,6 +44,7 @@ User Guide
    :maxdepth: 1
 
    AI Functions <../notebooks/generative_ai/ai_functions.ipynb>
+   AI Functions for Poster Analysis <../notebooks/generative_ai/ai_movie_poster.ipynb>
    AI Forecast <../notebooks/generative_ai/bq_dataframes_ai_forecast.ipynb>
    LLM Code Generation <../notebooks/generative_ai/bq_dataframes_llm_code_generation.ipynb>
    LLM KMeans <../notebooks/generative_ai/bq_dataframes_llm_kmeans.ipynb>