Document our AST shape and API

[char0n]

Full AST documentation needs to provided. Before we have one, please read below:

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Our AST is based on minim which is an implementation of the Refract Specification. We take it's types and extend them. Before we have proper AST documentation, the minim documentation is a good place to look at to understand our AST.

For example OpenAPI Info Object is represented as an extensions to minim Object Element with additional getters and setters.