Add docs for LLM Extension

[tgahunia05]

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[prrao87]

@ray6080 if this extension's purpose is only to provide embedding function calls from Cypher, shouldn't we call this an "embeddings extension"? LLM extension is quite broad and can mean different things to different people. Also, technically, speaking, LLMs do not do embedding (they are generative models trained on left-to-right token sequences -- embedding models are a totally different class of models trained via contrastive learning).

[prrao87]

Minor comments and questions.

[sdht0]

I'll take another look.

[sdht0]

Merge once the Python scripts are tested.