Initial GPU docs (#437)

* Initial GPU docs

* Apply suggestions from code review

Co-authored-by: Ekaterina Mekhnetsova <outoftardis@users.noreply.github.com>

* fixing comments

* Update doc/index.rst

Co-authored-by: Ekaterina Mekhnetsova <outoftardis@users.noreply.github.com>

* Update doc/index.rst

Co-authored-by: Ekaterina Mekhnetsova <outoftardis@users.noreply.github.com>

* Apply suggestions from code review

Co-authored-by: Ekaterina Mekhnetsova <outoftardis@users.noreply.github.com>

Co-authored-by: Ekaterina Mekhnetsova <outoftardis@users.noreply.github.com>

README.md

@@ -44,14 +44,14 @@ Core functioanlity of daal4py is in place Scikit-learn patching - Same Code, Sam
 Intel CPU optimizations patching
 ```py
 from daal4py.sklearn import patch_sklearn
+from daal4py.oneapi import sycl_context
 patch_sklearn()
 
-from sklearn.svm import SVC
-from sklearn.datasets import load_digits
-digits = load_digits()
-X, y = digits.data, digits.target
-clf = SVC().fit(X, y)
-res = clf.predict(X)
+from sklearn.cluster import DBSCAN
+
+X = np.array([[1., 2.], [2., 2.], [2., 3.],
+            [8., 7.], [8., 8.], [25., 80.]], dtype=np.float32)
+clustering = DBSCAN(eps=3, min_samples=2).fit(X)
 ```
 
 Intel CPU/GPU optimizations patching
@@ -60,14 +60,14 @@ from daal4py.sklearn import patch_sklearn
 from daal4py.oneapi import sycl_context
 patch_sklearn()
 
-from sklearn.svm import SVC
-from sklearn.datasets import load_digits
-digits = load_digits()
-X, y = digits.data, digits.target
+from sklearn.cluster import DBSCAN
+
+X = np.array([[1., 2.], [2., 2.], [2., 3.],
+            [8., 7.], [8., 8.], [25., 80.]], dtype=np.float32)
 with sycl_context("gpu"):
-    clf = SVC().fit(X, y)
-    res = clf.predict(X)
+    clustering = DBSCAN(eps=3, min_samples=2).fit(X)
 ```
+
 daal4py API, allows you to use wider set of Intel(R) oneAPI Data Analytics Library algorithms in just one line:
 ```py
 import daal4py as d4p

doc/index.rst

@@ -42,9 +42,8 @@ the linear regression workflow is showcased below::
 In the example above, it can be seen that model is divided into training and
 prediction.  This gives flexibility when writing custom grid searches and custom
 functions that modify model behavior or use it as a parameter. Daal4py also
-allows for direct usage of NumPy arrays and Pandas DataFrames instead of DAAL's
-NumericTables, which allow for better integration with the Pandas/NumPy/SciPy stack.
-
+allows for direct usage of NumPy arrays and pandas DataFrames instead of oneDAL
+NumericTables, which allow for better integration with the pandas/NumPy/SciPy stack.
 
 Daal4py machine learning algorithms are constructed with a rich set of
 parameters. Assuming we want to find the initial set of centroids for kmeans,
@@ -82,6 +81,43 @@ Last but not least, daal4py allows :ref:`getting input data from streams <stream
         algo.compute(input)
     result = algo.finalize()
 
+oneAPI and GPU support in daal4py
+---------------------------------
+daal4py provides support of oneAPI concepts such as context and queues, which means that
+algorithms can be executed on different devices, GPUs in particular. This is implemented via 'with sycl_context("xpu")'
+blocks that redirect execution to a device of the selected type: GPU, CPU, or host.
+Same approach is implemented for scikit-learn patching, so scikit-learn programs can be
+executed on GPU devices as well.
+
+To patch your code with Intel CPU/GPU optimizations:
+
+.. code-block:: python
+
+   from daal4py.sklearn import patch_sklearn
+   from daal4py.oneapi import sycl_context
+   patch_sklearn()
+
+   from sklearn.cluster import DBSCAN
+
+   X = np.array([[1., 2.], [2., 2.], [2., 3.],
+               [8., 7.], [8., 8.], [25., 80.]], dtype=np.float32)
+   with sycl_context("gpu"):
+      clustering = DBSCAN(eps=3, min_samples=2).fit(X)
+
+For execution on GPU, DPC++ compiler runtime and driver are required. Refer to `DPC++ system
+requirements <https://software.intel.com/content/www/us/en/develop/articles/intel-oneapi-dpcpp-system-requirements.html>`_ for details.
+
+DPC++ compiler runtime can be installed either from PyPI or Anaconda:
+
+- Install from PyPI::
+  
+     pip install dpcpp-cpp-rt
+
+- Install from Anaconda::
+  
+     conda install dpcpp_cpp_rt -c intel 
+
+
 Daal4py's Design
 ----------------
 The design of daal4py utilizes several different technologies to deliver Intel(R) oneAPI Data