docs: add example for reusing an existing collection (#32774)
Added a short section to the Weaviate integration docs showing how to
connect to an existing collection (reuse an index) with
`WeaviateVectorStore`. This helps clarify required parameters
(`index_name`, `text_key`) when loading a pre-existing store, which was
previously missing.
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### Description
Added a short section to the Weaviate integration docs showing how to
connect to an existing collection (reuse an index) with
`WeaviateVectorStore`. This helps clarify required parameters
(`index_name`, `text_key`) when loading a pre-existing store, which was
previously missing.
### Issue
Fixes langchain-ai/langchain-weaviate#197
### Dependencies
None
