fix: add include_bucket_in_prefix param to bulk exports documentation (#1753)

## Overview
<!-- Brief description of what documentation is being added/updated -->
Add documentation for recently added `include_bucket_in_prefix` optional
param when creating destinations for bulk exports
## Type of change

**Type:** Update existing documentation 

## Related issues/PRs
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- GitHub issue:
- Feature PR: langchain-ai/langchainplus#13772

<!-- For LangChain employees, if applicable: -->
- Linear issue:
https://linear.app/langchain/issue/ENT-210/avoid-none-prefix-in-bulk-export-destination-path
- Slack thread:
https://langchain.slack.com/archives/C09GLF9F96D/p1764940224259009?thread_ts=1764940071.452319&cid=C09GLF9F96D

## Checklist
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- [x] I have read the [contributing guidelines](README.md)
- [x] I have tested my changes locally using `docs dev`
- [x] All code examples have been tested and work correctly
- [x] I have used **root relative** paths for internal links
- [x] I have updated navigation in `src/docs.json` if needed

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## Additional notes
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Author: kenvora

src/langsmith/data-export.mdx

@@ -36,6 +36,7 @@ The following information is needed for the export:
 - **Endpoint URL**: The endpoint URL for the S3 bucket - this is needed for S3 API compatible buckets.
 - **Access Key**: The access key for the S3 bucket.
 - **Secret Key**: The secret key for the S3 bucket.
+- **Include Bucket in Prefix** (optional): Whether to include the bucket name as part of the path prefix. Defaults to `false` for new destinations or when the bucket name is already present in the path. Set to `true` for legacy compatibility or when using storage systems that require the bucket name in the path.
 
 We support any S3 compatible bucket, for non AWS buckets such as GCS or MinIO, you will need to provide the endpoint URL.
 
@@ -74,7 +75,8 @@ curl --request POST \
       "bucket_name": "your-s3-bucket-name",
       "prefix": "root_folder_prefix",
       "region": "your aws s3 region",
-      "endpoint_url": "your endpoint url for s3 compatible buckets"
+      "endpoint_url": "your endpoint url for s3 compatible buckets",
+      "include_bucket_in_prefix": true
     },
     "credentials": {
       "access_key_id": "YOUR_S3_ACCESS_KEY_ID",
@@ -143,6 +145,7 @@ curl --request POST \
       "bucket_name": "my_bucket",
       "prefix": "data_exports",
       "endpoint_url": "https://storage.googleapis.com"
+      "include_bucket_in_prefix": true
     },
     "credentials": {
       "access_key_id": "YOUR_S3_ACCESS_KEY_ID",