[Docs] RayDP Documentation (#14018)

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* Update raydp.rst

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Co-authored-by: Alex Wu <alex@anyscale.com>

doc/source/index.rst

@@ -304,6 +304,7 @@ Papers
    xgboost-ray.rst
    dask-on-ray.rst
    mars-on-ray.rst
+   raydp.rst
    ray-client.rst
 
 .. toctree::

doc/source/raydp.rst

@@ -0,0 +1,104 @@
+********************
+RayDP (Spark on Ray)
+********************
+
+RayDP combines your Spark and Ray clusters, making it easy to do large scale
+data processing using the PySpark API and seemlessly use that data to train
+your models using TensorFlow and PyTorch.
+
+For more information and examples, see the RayDP Github page:
+https://github.com/oap_project/raydp
+
+================
+Installing RayDP
+================
+
+RayDP can be installed from PyPI and supports PySpark 3.0 and 3.1.
+
+.. code-block bash
+
+  pip install raydp
+
+.. note::
+  RayDP requires ray >= 1.2.0
+
+.. note::
+  In order to run Spark, the head and worker nodes will need Java installed.
+
+========================
+Creating a Spark Session
+========================
+
+To create a spark session, call ``raydp.init_spark``
+
+For example,
+
+.. code-block:: python
+
+  import raydp
+
+  spark = raydp.init_spark(
+    app_name = "example",
+    num_executors = 10,
+    executor_cores = 64,
+    memory_per_executor = "256GB"
+  )
+
+====================================
+Deep Learning with a Spark DataFrame
+====================================
+
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+Training a Spark DataFrame with TensorFlow
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+``raydp.tf.TFEstimator`` provides an API for training with TensorFlow.
+
+.. code-block:: python
+
+  d = [{'age': 17 , 'grade': 12}]
+  df = spark.createDataFrame(d).collect()
+
+
+  from tensorflow import keras
+  model = keras.Sequential([])
+
+  estimator = raydp.tf.TFEstimator(
+    model = model,
+    num_worker = 10,
+    feature_columns = ["age"],
+    label_column = ["grade"]
+  )
+
+  estimator.fit_on_spark(df, test_df=None)
+
+  tensorflow_model = estimator.get_model()
+
+
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+Training a Spark DataFrame with TensorFlow
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Similarly, ``raydp.torch.TorchEstimator`` provides an API for training with
+PyTorch.
+
+.. code-block:: python
+
+  d = [{'age': 17 , 'grade': 12}]
+  df = spark.createDataFrame(d).collect()
+
+
+  import torch
+  model = torch.nn.Sequential()
+
+  estimator = raydp.tf.TFEstimator(
+    model = model,
+    num_worker = 10,
+    feature_columns = ["age"],
+    label_column = ["grade"]
+  )
+
+  estimator.fit_on_spark(df, test_df=None)
+
+  pytorch_model = estimator.get_model()
+