`public-chain-data` miscategorized as a native function in the docs

[LindaOrtega]

public-chain-data appears as a native function in the docs (see https://pact-language.readthedocs.io/en/latest/pact-functions.html#public-chain-data). When it's actually the schema type for the chain-data native function.

[emilypi]

Thanks for finding this. There are 2 fixes for this of varying degrees of difficulty:

1. We include a section in the "Built-in Functions" section of the readthedocs for Schema, which amounts to included the schema in a separate NativeModule. This is the easy fix.

2. The harder fix would be to go in Docgen.hs and hack in support for a new section in the docs for built-in datatypes such as schema.