feat: add Nhost example vector database

examples/vector_databases/nhost/README.md

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+# What is Nhost?
+
+[Nhost](https://nhost.io/) is an enterprise-grade open-source backend platform built on top of GraphQL & PostgreSQL. Nhost offers the convenience of Firebase, with the extensibility of platforms like Render and Heroku.
+
+Nhost is a great choice for full-stack teams that want to get out of the ground fast, but also want to have the flexibility to customize their backend as they grow.
+
+## Vector search and PostgreSQL
+
+Nhost supports vector search using the [pgvector](https://docs.nhost.io/guides/database/extensions#pgvector) open-source PostgreSQL extension. With `pgvector`, you can store and query high-dimensional vectors alongside your application data.
+
+In this guide, we leverage Graphite, an AI service from the Nhost stack that automatically generates and keeps your embeddings up-to-date.
+
+### Semantic search with Nhost Postgres and OpenAI
+
+In this notebook you will learn how to:
+
+1. Run a development instance of Nhost Postgres with the `pgvector` extension.
+2. Leverage Auto-Embeddings & OpenAI to generate and keep your embeddings up-to-date automatically.
+3. Perform semantic search using natural language and GraphQL.
+
+## Additional Resources
+
+- [Nhost AI product page](https://nhost.io/product/graphite)
+- [Nhost AI documentation](https://docs.nhost.io/product/ai)
+- [pgvector GitHub repository](https://github.com/pgvector/pgvector)