docs: Update getting_started.ipynb link to correct jupyter notebook path in README.md

[iamarunbrahma]

What does this PR do?

This PR fixes a broken link in the README.md that was causing a 404 error. The link to getting_started.ipynb was pointing to a non-existent file. Updated it to point to the correct notebook Llama_Stack_Building_AI_Applications.ipynb which contains the walk-through for text and vision inference llama_stack_client APIs.

• Addresses issue (getting_started.ipynb link on llama.com is a 404 #633 )

Test Plan

1. Verified that the new notebook path exists:

   ls docs/notebooks/Llama_Stack_Building_AI_Applications.ipynb

2. Verified the notebook content contains text and vision inference examples by:

   • Checking the notebook contents
   • Confirming the presence of vision models like Llama-3.2-11B-Vision-Instruct
   • Verifying llama_stack_client API usage examples

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Ran pre-commit to handle lint / formatting issues.
• Read the contributor guideline, Pull Request section.
• Updated relevant documentation.
• Wrote necessary unit or integration tests (N/A - documentation change only).

[ashwinb]

thank you!