added advanced index

doc/advanced/function.txt

+
+.. _function:
+
+==================
+function interface
+==================
+
+WRITEME
+

doc/advanced/index.txt

+
+.. _advanced:
+
+====================================
+Advanced Topics (under construction)
+====================================
+
+.. toctree::
+    :maxdepth: 2
+   
+    env
+    features
+    optimization
+    compilation
+    ccodegen
+    function
+    module
+