docs: add milvus multitenancy doc (#16177)
- **Description:** add milvus multitenancy doc, it is an example for
this [pr](https://github.com/langchain-ai/langchain/pull/15740) .
- **Issue:** No,
- **Dependencies:** No,
- **Twitter handle:** No
Signed-off-by: ChengZi <chen.zhang@zilliz.com>
