feat: add Nhost example vector database

[nunopato]

Summary

Adds a cookbook example to store and query OpenAI embeddings in Nhost using pgvector and Auto-Embeddings, a feature that makes it trivial to keep OpenAI embeddings up-to-date.

Motivation

Nhost users are using Auto-Embeddings to keep their embeddings up-to-date with minimal effort, while performing semantic search over their data through generated GraphQL APIs. This PR adds a new guide that showcases a novel way to keep embeddings up-to-date without having to write any code.

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