docs: batch embeddings (Fixes #147)

[aparna534]

Description

This adds a working example of batch text embeddings using genai.embed_content().

Key Changes:

• New file: examples/gemini/python/batched_embeddings.py
  • It processes multiple texts in a single API call
  • It uses environment variables for API key configuration
  • It includes comprehensive error handling
• Updated: examples/gemini/python/README.md with instructions

Verification:

# Local test output

Success! Generated 5 embeddings:
Text 1: First 5 dims: [0.053, -0.001, -0.065, -0.005, 0.026]
Text 2: First 5 dims: [0.045, -0.028, -0.061, 0.012, 0.011]

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[aparna534]

@googlebot I signed the CLA