change link of JAX AI Libraries to Cloud docs

[jacoguzo]

Description

Changes link in README.md from https://docs.jaxstack.ai/en/latest/getting_started.html to https://docs.cloud.google.com/tpu/docs/jax-ai-stack

Checklist

Before submitting this PR, please make sure (put X in square brackets):

• I have performed a self-review of my code. For an optional AI review, add the gemini-review label.
• I have necessary comments in my code, particularly in hard-to-understand areas.
• I have run end-to-end tests tests and provided workload links above if applicable.
• I have made or will make corresponding changes to the doc if needed, including adding new documentation pages to the relevant Table of Contents (toctree directive) as explained in our documentation.