Add Astra DB (/Cassandra) to vector databases with example notebooks (#655)

* first commit for the Astra DB / Cassandra notebooks

* add json with quotes

* towards the final cassIO pilot notebook

* small changes to the copy

* cassIO pilot completed and its readme done

* fix silly markdown error

* fix silly markdown error 2

* astra vector link change and vector search picture improved

* added link to docs for connecting to Cassandra cluster

* CQL version of the flow

* revised readme

* final adjustments around metrics/distances

* links' final version assumes files in openai's main branch

* Add ref to QA+vector general guide; fix prompt; clarified conclusion paragraph; typos

examples/vector_databases/cassandra_astradb/README.md

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+# Cassandra / Astra DB
+
+The example notebooks in this directory show how to use the Vector
+Search capabilities available today in **DataStax Astra DB**, a serverless
+Database-as-a-Service built on Apache Cassandra®.
+
+Moreover, support for vector-oriented workloads is making its way to the
+next major release of Cassandra, so that the code examples in this folder
+are designed to work equally well on it as soon as the vector capabilities
+get released.
+
+If you want to know more about Astra DB and its Vector Search capabilities,
+head over to [astra.datastax.com](https://docs.datastax.com/en/astra-serverless/docs/vector-search/overview.html) or try out one
+of these hands-on notebooks straight away:
+
+### Example notebooks
+
+The following examples show how easily OpenAI and DataStax Astra DB can
+work together to power vector-based AI applications. You can run them either
+with your local Jupyter engine or as Colab notebooks:
+
+| Use case | Framework | Notebook | Google Colab |
+| -------- | --------- | -------- | ------------ |
+| Search/generate quotes | CassIO | [Notebook](./Philosophical_Quotes_cassIO.ipynb) | [![Colab](https://colab.research.google.com/assets/colab-badge.svg)](https://colab.research.google.com/github/openai/openai-cookbook/blob/main/examples/vector_databases/cassandra_astradb/Philosophical_Quotes_cassIO.ipynb) |
+| Search/generate quotes | Plain Cassandra language | [Notebook](./Philosophical_Quotes_CQL.ipynb) | [![Colab](https://colab.research.google.com/assets/colab-badge.svg)](https://colab.research.google.com/github/openai/openai-cookbook/blob/main/examples/vector_databases/cassandra_astradb/Philosophical_Quotes_CQL.ipynb) |
+
+### Vector similarity, visual representation
+
+![3_vector_space](https://user-images.githubusercontent.com/14221764/262321363-c8c625c1-8be9-450e-8c68-b1ed518f990d.png)