Update GPU admonition to match jstac's text (#448)

* Add centralized GPU admonition for JAX lectures

- Create _admonition/gpu.md for single-source GPU notice
- Update jax_intro.md to use include directive
- Update numpy_vs_numba_vs_jax.md to use include directive

This makes it easier to maintain consistent GPU notices across
all JAX-related lectures.

* Fix grammar: target -> targets

* Simplify GPU admonition text

* Update GPU admonition to match jstac's text from lecture-python.myst

Author: mmcky

lectures/_admonition/gpu.md

@@ -1,5 +1,12 @@
 ```{admonition} GPU
 :class: warning
 
-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](https://github.com/google/jax).
+This lecture was built using a machine with access to a GPU.
+
+[Google Colab](https://colab.research.google.com/) has a free tier with GPUs
+that you can access as follows:
+
+1. Click on the "play" icon top right
+2. Select Colab
+3. Set the runtime environment to include a GPU
 ```