[Doc] doc page for builtin functions (#642)

* doc page for builtin functions; install from source doc

* upload rst

* update

docs/source/index.rst

@@ -163,6 +163,14 @@ The model tutorials are categorized based on the way they utilize DGL APIs.
 
 Or go through all of them :doc:`here <tutorials/models/index>`.
 
+.. toctree::
+   :maxdepth: 1
+   :caption: Features
+   :hidden:
+   :glob:
+
+   features/builtin
+
 .. toctree::
    :maxdepth: 3
    :caption: Model Tutorials