Add "preview in JupyterLite" button to README

[psychemedia]

Is there a reason for not having a button that launches a demo environment with jupyterlab-myst pre-installed using JupyterLite at the top of the README file?

The Launch in Binder experience rarely works for me any more (hmmm, although trying Binder in other browsers just now, this may be a probably with either: a) Chrome, or b) one of the extensions I have installed in Chrome)?

But aside from that, JuptyerLite was pushed as a way of bringing docs alive, so it feels strange not for it to be used to demonstrate this extension.