Suggestion: Add basic progress tracking to website

[eliasm307]

Problem

The examples are great, however they are not completable in one sitting. So if someone goes through them over time, in multiple sittings, it might get difficult to remember where they stopped when next opening the site

Proposal

Track which examples a user has "completed", and show this in the UI, e.g. a checkmark next to the examples.

For a basic implementation, completion could just mean the user has viewed the example and pressed the "next" button.

The completion state can be stored in local storage for persistence.