docs: Vector store notebook docs (googleapis#18)

* feat: Adding Vector Store Implementation for Spanner

- Static Utility to create a table to store vector embeddings
- Automatic Dialect Detection for GSQL and PG
- Similarity Search & Maximal Marginal Relevance Search
- Add documents and texts

* feat: Validating Types, Columns at the time of VectorStore initialization & also while creating new table through static utility.

* feat: Delete Vector embeddings through documents.

* style: format and lint

* feat: Adding init file to export VectorStore

* feat: Adding delete to dynamically generate SQL for each row.

* feat: Adding full support for staleness with multiple options like read_timestamp and min and exact staleness.

* feat: Addressing review comments and adding data class for indexes.

* feat: Adding support for secondary indexes.

* feat: exporting secondary indexes.

* style: lint

* feat: Initiating client with user agent.

* feat: Addressing review comments.

* style: lint

* style: lint

* style: lint

* docs: Implementing jupyter notebook for vector store

* Adding story for deletion and using VertexAIEmbeddings model.

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Co-authored-by: Averi Kitsch <[email protected]>

Author: gauravpurohit06