Indexing the notebook showing how to reproduce the GIF. (#640)

Co-authored-by: Junpeng Lao <junpenglao@gmail.com>

docs/index.md

@@ -114,6 +114,7 @@ Sample with multiple chains?<examples/howto_sample_multiple_chains.md>
 Use custom gradients?<examples/howto_custom_gradients.md>
 Use non-JAX log-prob functions?<examples/howto_other_frameworks.md>
 Build a Metropolis-Within-Gibbs sampler?<examples/howto_metropolis_within_gibbs.md>
+Sample from the word BlackJAX using BlackJAX?<examples/howto_reproduce_the_blackjax_image.md>
 ```
 
 ```{toctree}