Add documentation for preview module (uxlfoundation#2155)

Author: Alexsandruss

doc/sources/index.rst

@@ -109,6 +109,7 @@ Enable Intel(R) GPU optimizations
    input-types.rst
    array_api.rst
    verbose.rst
+   preview.rst
    deprecation.rst
 
 

doc/sources/preview.rst

@@ -0,0 +1,76 @@
+.. ******************************************************************************
+.. * Copyright 2024 Intel Corporation
+.. *
+.. * Licensed under the Apache License, Version 2.0 (the "License");
+.. * you may not use this file except in compliance with the License.
+.. * You may obtain a copy of the License at
+.. *
+.. *     http://www.apache.org/licenses/LICENSE-2.0
+.. *
+.. * Unless required by applicable law or agreed to in writing, software
+.. * distributed under the License is distributed on an "AS IS" BASIS,
+.. * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+.. * See the License for the specific language governing permissions and
+.. * limitations under the License.
+.. *******************************************************************************/
+
+.. _preview:
+
+#####################
+Preview Functionality
+#####################
+
+Some of implemented functionality in |intelex| doesn't meet one or few of next requirements
+for being enabled by default for all users:
+
+* The functionality API is not stable and can be changed in future
+* The functionality doesn't have full compatibility with stock Scikit-learn
+* The functionality misses performance targets compared to stock Scikit-learn or previously available version of functionality
+* The functionality is not fully tested
+
+This type of functionality is available under **preview mode** of |intelex| and located in
+the corresponding module (`sklearnex.preview`).
+
+Preview functionality *may* or *may not* participate in patching of Scikit-learn.
+For example, a preview estimator may be a replacement for a stock one or a completely new one.
+
+To enable preview functionality, you need to set the `SKLEARNEX_PREVIEW` environment variable
+to non-empty value before patching of Scikit-learn.
+For example, you can set the environment variable in the following way:
+
+- On Linux* OS ::
+
+     export SKLEARNEX_PREVIEW=1
+
+- On Windows* OS ::
+
+     set SKLEARNEX_PREVIEW=1
+
+Then, you can import Scikit-learn estimator patched with a preview one from `sklearnex.preview` module::
+
+     from sklearnex import patch_sklearn
+     patch_sklearn()
+     from sklearn.linear_model import Ridge
+     print(Ridge.__module__)
+     # output:
+     # sklearnex.preview.linear_model.ridge
+
+Current list of preview estimators:
+
+.. list-table::
+   :widths: 30 20 10
+   :header-rows: 1
+   :align: left
+
+   * - Estimator name
+     - Module
+     - Is patching supported
+   * - EmpiricalCovariance
+     - sklearnex.preview.covariance
+     - Yes
+   * - IncrementalPCA
+     - sklearnex.preview.decomposition
+     - Yes
+   * - Ridge
+     - sklearnex.preview.linear_model
+     - Yes