Proposal: Modernizing Generative AI README and fixing grammatical inconsistencies

### Description I would like to propose an update to the `generative_ai/README.md` file. The current documentation contains some grammatical inconsistencies in the "Getting Started" section and refers to older model versions (Gemini 1.5). ### Proposed Changes 1. **Grammar Fix:** Correct the sentence structure in the "Getting Started" section to improve readability. 2. **Modernization:** Update the feature table to reflect 2026 standards and ensure model references are current. 3. **Security:** Add a brief note about credential management best practices (ADC/Secret Manager) for users running samples locally. I have already prepared the changes in a fork and I'm ready to submit a Pull Request if this proposal is accepted.