Split tutorial into smaller pieces to make learning easier

Currently, the [Jsonnet tutorial](https://jsonnet.org/learning/tutorial.html) is a single long page with all of the functionality. It doesn't have a table of contents, or step-by-step mode, which means the ways to consume it are:

* scroll through the whole thing, top down
* search for items of interest via Ctrl-F

As an alternative, let's consider [A Tour of Go](https://tour.golang.org/), which provides step-by-step introduction to the language, with a live try-it-in-your-browser evaluation. Even if that goal is hard to reach in one step, the following incremental improvements would help:

1. create a sticky table of contents on the left that show available modules, that link to the appropriate part of the page
2. create a separate page per tutorial element, so each subpage is short and easy to consume (with next/previous navigation)
3. finally, add a scratch pad for live evaluation, like in the Go tour, for folks to try it out in context

Thoughts?