Improve GPU admonition with clearer Colab instructions (#750)

Simplified the GPU admonition to focus on practical steps for accessing
GPU resources through Google Colab. Removed technical details about CUDA
and CUDANN that were not actionable for most users, and restructured the
Colab access instructions as a numbered list for better clarity.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude <noreply@anthropic.com>

Author: jstac

lectures/_admonition/gpu.md

@@ -1,9 +1,12 @@
 ```{admonition} GPU
 :class: warning
 
-This lecture was built using a machine with the latest CUDA and CUDANN frameworks installed with access to a GPU.
+This lecture was built using a machine with access to a GPU.
 
-To run this lecture on [Google Colab](https://colab.research.google.com/), click on the "play" icon top right, select Colab, and set the runtime environment to include a GPU.
+[Google Colab](https://colab.research.google.com/) has a free tier with GPUs
+that you can access as follows:
 
-To run this lecture on your own machine, you need to install the software listed following this notice.
+1. Click on the "play" icon top right
+2. Select Colab
+3. Set the runtime environment to include a GPU
 ```