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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For new content

When contributing new content, read through our contribution guidelines, and mark the following action items as completed:

• [x] I have added a new entry in registry.yaml (and, optionally, in authors.yaml) so that my content renders on the cookbook website.
• [x] I have conducted a self-review of my content based on the contribution guidelines:
  • [x] Relevance: This content is related to building with OpenAI technologies and is useful to others.
  • [x] Uniqueness: I have searched for related examples in the OpenAI Cookbook, and verified that my content offers new insights or unique information compared to existing documentation.
  • [x] Spelling and Grammar: I have checked for spelling or grammatical mistakes.
  • [x] Clarity: I have done a final read-through and verified that my submission is well-organized and easy to understand.
  • [x] Correctness: The information I include is correct and all of my code executes successfully.
  • [x] Completeness: I have explained everything fully, including all necessary references and citations.

[nunopato]

It would be nice to get some eyes and feedback here, thank you! /cc @simonpfish @ibigio

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This PR was closed because it has been stalled for 10 days with no activity.