Fix additional build warnings (#57)

- getting_started.md: Fix index directive missing required argument (line 95)
- _admonition/gpu.md: Add missing GPU admonition file for jax_intro.md and numpy_vs_numba_vs_jax.md includes

Author: mmcky

lectures/_admonition/gpu.md

@@ -0,0 +1,12 @@
+```{admonition} GPU
+:class: warning
+
+This lecture was built using a machine with access to a GPU --- although it will also run without one.
+
+[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
+```

lectures/getting_started.md

@@ -90,12 +90,15 @@ kernelspec:
 برای اطلاعات بیشتر درباره کوندا، در ترمینال عبارت " conda help " را تایپ کنید.
 
 (ipython_notebook)=
-## Jupyter Notebooks
+## {index}`Jupyter Notebooks <single: Jupyter Notebooks>`
 
-```{index}
-single: Python; IPython
-single: IPython
-single: Jupyter
+```{index} single: Python; IPython
+```
+
+```{index} single: IPython
+```
+
+```{index} single: Jupyter
 ```
 
 [ژوپیتر](http://jupyter.org/) نوت بوک ها یکی از چندین راه های قابل دسترس برای ارتباط برقرار کردن با پایتون هستند.