Add progress meter functionality

[ASKabalan]

Answers #134

[ASKabalan]

BTW I just copy pasted the progress bar from Diffrax

Let me know if you are interested in a more general progress bar implementation for JAX that supports multiple bars with vmap and shardmap => https://github.com/ASKabalan/jbar

[ASKabalan]

@patrick-kidger I fixed the mkdocs error

the workflow should work now

Thank you

[patrick-kidger]

Thanks for the PR! Sorry that this took me so long to get to 😅

So this kind of functionality is something I would put in equinox.internal: it's not large enough for me to spin it out into its own library (as you've done!), but I also want to avoid duplicating it amongst the libraries that I have.

Would you be interested in opening a PR adding this to Equinox (+then pointing Optimistix at it + refactoring Diffrax to use it)? If so then I'd be very happy to support progress bar functionality in Optimistix. :)