box: add retrievers and fix docs (#25633)

Thank you for contributing to LangChain!


**Description:** Adding `BoxRetriever` for langchain_box. This retriever
handles two use cases:
* Retrieve all documents that match a full-text search
* Retrieve the answer to a Box AI prompt as a Document

**Twitter handle:** @BoxPlatform


- [x] **Add tests and docs**: If you're adding a new integration, please
include
1. a test for the integration, preferably unit tests that do not rely on
network access,
2. an example notebook showing its use. It lives in
`docs/docs/integrations` directory.


- [x] **Lint and test**: Run `make format`, `make lint` and `make test`
from the root of the package(s) you've modified. See contribution
guidelines for more: https://python.langchain.com/docs/contributing/

Additional guidelines:
- Make sure optional dependencies are imported within a function.
- Please do not add dependencies to pyproject.toml files (even optional
ones) unless they are required for unit tests.
- Most PRs should not touch more than one package.
- Changes should be backwards compatible.
- If you are adding something to community, do not re-import it in
langchain.

If no one reviews your PR within a few days, please @-mention one of
baskaryan, efriis, eyurtsev, ccurme, vbarda, hwchase17.

---------

Co-authored-by: Erick Friis <erick@langchain.dev>

docs/docs/integrations/providers/box.mdx

@@ -172,3 +172,14 @@ If you wish to use OAuth2 with the authorization_code flow, please use `BoxAuthT
 from langchain_box.document_loaders import BoxLoader
 
 ```
+
+## Retrievers
+
+### BoxRetriever
+
+[See usage example](/docs/integrations/retrievers/box)
+
+```python
+from langchain_box.retrievers import BoxRetriever
+
+```