Add ChainForge tool to `related_resources.md`

[ianarawjo]

Summary

Adds the academic open-source project ChainForge, which helps users battle-test prompts and model choices in a visual data flow UI.

Motivation

Why are these changes necessary? How do they improve the cookbook?

ChainForge is one of the few low/no-code tools that lets you set up complex evaluations and comparisons entirely in a visual front-end UI, and has several published research papers, such as "Who Validates the Validators?" and ChainBuddy.

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