Add some reference material on golang Context

Both of our Cohort 2 trainees have raised existential questions around Context. We may want to find some useful references or write up some material about this.

We may even want to make a "implement `WithTimeout` using `WithValue`" exercise to show that there's no magic here, it's really just a fancy key-value store.

(I've also gone on a tangent around [persistent data structures](https://en.wikipedia.org/wiki/Persistent_data_structure) when talking through this, unclear whether that's actually useful/worthwhile :))