[Doc] Add Data Plane access documentation (#732)

## Changes
Add Data Plane access documentation

## Tests
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Author: hectorcast-db

docs/dataplane.md

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+# Data Plane APIs
+
+Some APIs such as Model Serving support direct Data Plane access for higher throughput and lower latency requests. 
+To access Data Plane access, a dedicated short-lived OAuth token must be used. The SDK is able to generate and refresh 
+such tokens transparently for the user.
+
+## Prerequisites
+Databricks SDK must be configured using a supported OAuth token. For more information, see
+[Supported Databricks authentication types](https://docs.databricks.com/en/dev-tools/auth/index.html)
+
+The desired service or endpoint must have direct Data Plane access enabled.
+
+## Usage
+Databricks SDK provides a separate service to be used for Data Plane access, which includes a `_data_plane` suffix.
+This service contains the subset of the methods for the original service which are supported in the Data Plane.
+
+Example:
+
+```python
+from databricks.sdk import WorkspaceClient
+# Control Plane
+w = WorkspaceClient()
+w.serving_endpoints.query(...)
+# Data Plane
+w.serving_endpoints_data_plane.query(...)
+```
+