Add Google Cloud SQL for PostgreSQL integration documentation (#17813)

docs/docs/community/integrations/vector_stores.md

docs/docs/examples/chat_store/CloudSQLPgChatStoreDemo.ipynb

.../docs/examples/data_connectors/CloudSQLPgReaderDemo.ipynb

docs/docs/examples/docstore/CloudSQLPgDocstoreDemo.ipynb

...cs/examples/vector_stores/CloudSQLPgVectorStoreDemo.ipynb

docs/docs/module_guides/loading/connector/modules.md

docs/docs/module_guides/storing/chat_stores.md

docs/docs/module_guides/storing/docstores.md

docs/docs/module_guides/storing/index_stores.md

docs/docs/module_guides/storing/vector_stores.md