Document how homotopy.io works

[NickHu]

Currently, the best description of what is going on in homotopy.io is https://arxiv.org/abs/1902.03831. However, this is targeted towards an academic audience and is at a higher level of abstraction than what is going on here. It would be nice to have a less formal document which describes the components and how everything works together, and is more implementation-focused.