Update OpenSlide homepage to a more modern design

[logankilpatrick]

Would it be helpful for the project to have an updated home page which is more modern? I just spun up https://sciml.ai recently and would be happy to do an updated version of this site as well. I can post some possible samples here with different looks if there's interest in this!

[bgilbert]

Yes, absolutely! A refresh is overdue. The current site began life as a snapshot of an old Trac wiki.

I'd suggest that a new site should:

• Implement the design with Jekyll layouts so we don't need to do a rendering pass with a separate tool.
• Preserve existing links as much as possible, and perform redirects where it can't. This may not be practical for the front page (and maybe the download page), which is sort of a mess.