Add story mode / feature tour

[sharmaeklavya2]

Currently the game starts with a level generated by bp1. We have more levels and generators, but the user may not know that there's more to explore or which levels to try out first. We should make changes to the UI to fix this.

One option is to add a 'story mode', where users are presented a curated list of levels one-by-one.

Another option is to add a 'feature tour' or tutorial, which highlights features like levels/generators, algorithms, zoom, save, edit.