docs: Clean-up index.md

[EwoutH]

Clean up index.md, updating the screenshot to a modern SolaraViz one and removing unnecessary information, leading to a clean landing page.

A rendered version of the updated landing page can be viewed here: https://mesa--2422.org.readthedocs.build/2422/

[quaquel]

This is a nice cleanup.

Note that in #2416, I point out a small bug in the existing grid visualization, which might require us to update the png before the final release (but this can be handled in seperate follow up PRs).