Add demo web page

[kaaveland]

This is raw and needs more work still. I aim to close #74 with this.

Currently looks like this:

https://github.com/kaaveland/eugene/assets/27794/7e1c6d03-89b3-4734-8752-adb4fae88e57

As part of this:

• Introduce more structure in eugene-web by making webapp-module with submodule for each handler
• Set up api routes in eugene_web::webapp::routes(), letting the location be passed from the outside
  • This isn't done yet, as index.hbs needs to have the path forwarded from the outside
• Pull out the splitting scripts thing into its own module -- pondering to move this to eugene

Still todo:

• [ ] - Add button to fetch new random example script sequence (should take the ones from doc)
• [x] - It's 2024, we need some styling (perhaps we can steal mdBook from /eugene/docs on the same host)
  • Would be even better if we could maybe just add a page to the mdBook and use htmx from there for the demo?

[kaaveland]

I'm going to skip the fetching random examples part for now.