Add centralized GPU admonition for JAX lectures

[mmcky]

Summary

This PR adds a centralized GPU admonition file that can be included in all JAX-related lectures, making it easier to maintain consistent notices across the repository.

Changes

1. New file: lectures/_admonition/gpu.md

   • Contains the standard GPU admonition text
   • Can be updated in one place to affect all JAX lectures

2. Updated jax_intro.md

   • Replaced inline GPU admonition with {include} _admonition/gpu.md

3. Updated numpy_vs_numba_vs_jax.md

   • Replaced inline GPU admonition with {include} _admonition/gpu.md

GPU Admonition Content

The new simplified admonition:

This lecture is designed to run on a GPU. To use Google Colab's free GPUs, 
click the play icon top right, select Colab, and set the runtime to include a 
GPU. For local GPU setup, see the JAX installation guide.

Benefits

• Single source of truth for GPU notice content
• Easier to update wording across all lectures
• Consistent with approach used in lecture-python.myst and lecture-jax
• Simplified, more concise messaging

[github-actions]

🚀 Deployed on https://692cb7d8556ae76a3d645bdb--epic-agnesi-957267.netlify.app

[mmcky]

@jstac what do you think about this simplification of text.