Document gathering custom data with the local datagatherer

Currently Preflight has a set of built-in datagatherers. This is a limitation, because if someone wants to create a new package but doesn't have a datagatherer that provides the information for it, the only option is to write Go code and create a new datagatherer. This might not be ideal for some people.

Ideally, there should be a way of plug into Preflight any external data provided from an arbitrary process or file or whatever that is generic.

Start by reading Preflight's built-in datagatherers and the current Go-based process for creating a new datagatherer. Determine how external data from an arbitrary process or file could be connected without new Go code. Done should be documented support for a generic custom-data integration, with its input and usage clearly explained.

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