Document `contextValue` recipe for circular references in context object

The `dataSources` configuration in <=AS3 provided a magical / easy path to providing the entire context object to data sources (in a circular reference manner) so that data sources could easily reference other bits on context as well as other data sources.

This is a bit of a sticking point for users, I imagine an example implementing a `Context` class of sorts would be helpful.

Start by reviewing the existing documentation around the `dataSources` configuration and `contextValue`. Add a focused `Context` class example showing how the context object and data sources can reference one another, and make the recipe clear enough for users to reproduce the circular-reference setup.

Written by the indexing model from the issue text.