Add document for geo-shape decimation

(cherry picked from commit 910fc6b2)

docs/tutorial/GeoShapeDecimation.md

+In order to improve indexing and search performance for documents with large geometry, the geo-shape of the following 
+GeoJSON types in the original shape attribute and virtual shape attribute if exists are decimated 
+by implementing Ramer–Douglas–Peucker algorithm:
+- LineString
+- MultiLineString
+- Polygon
+- MultiPolygon  
+
+In order to reduce the risk when extended evaluation of the solution is still on going, a feature flag that is managed by 
+the Partition Service is applied to the solution.
+Here is an example to enable this feature by setting the property "indexer-decimation-enabled" in a given data partition:
+```
+{
+   "indexer-decimation-enabled": {
+        "sensitive": false,
+        "value": "true"
+    }
+}
+```
+
+If the property "indexer-decimation-enabled" is not created or the property value is set to "false" (String type) in the 
+given data partition, the geo-shape decimation will be ignored.