Add Cassandra/Astra DB to the vector databases README (#665)

* Add Cassandra/Astra DB to the vector databases overall README

* Linking to a basic quickstart instead

examples/vector_databases/README.md

@@ -8,6 +8,7 @@ Each provider has their own named directory, with a standard notebook to introdu
 
 ## Guides & deep dives
 - [AnalyticDB](https://www.alibabacloud.com/help/en/analyticdb-for-postgresql/latest/get-started-with-analyticdb-for-postgresql)
+- [Cassandra/Astra DB](https://docs.datastax.com/en/astra-serverless/docs/vector-search/qandasimsearch-quickstart.html)
 - [Chroma](https://docs.trychroma.com/getting-started)
 - [Elasticsearch](https://www.elastic.co/guide/en/elasticsearch/reference/current/knn-search.html)
 - [Hologres](https://www.alibabacloud.com/help/en/hologres/latest/procedure-to-use-hologres)