docs: update readme with March 2026 model releases and pricing

- Add March 2026 model releases section with GPT-5.4 family models
- Update OpenAI pricing and model names (GPT-5.4, GPT-5.4 mini, GPT-5.4 nano)
- Refresh latest updates table with March 2026 releases
- Add update log table with version history
- Update last updated timestamp and document version to 1.6
- Normalize date formats and table structures throughout
- Update attribution and verification sources

Author: Shaerif