[WIP] Add `Scorer` Docs

[NiWaRe]

This is a WIP docs PR to:

• add an example LLM Correctness Judge by overwriting the weave.flow.scorers.Scorer class to the RAG tutorial
• add API docs for Scorer
• add some general best practices to defining scorers

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[scottire]

I pulled from master and it has too many changes now. Created https://github.com/wandb/weave/pull/1720 to replace this