Add guide on how to build demos for the Flax sprint
Add a guide on how to build a demo for the Flax sprint. This guide will be also updated early next week, but I would like to submit this version on Friday.

Note: this will be extended more once the evaluation is a bit more defined and with the launch of S.
Some TODO ideas (for a later iteration)
* Add small guide on Streamlit
* Add small guide on Gradio
* Add small guide on how to use S (or a video)
* Explain why the importance of a demo<|||||>I'll merge this today. I'll do a follow up PR on Monday extending the demo section.