Improve specification with mention of BESS-UPF

Additionally adding BESS-UPF page in advanced feature, poiting to Aether doc for now.

Change-Id: I93efcb78359fbc0980d995014dc75030bfcd5019
diff --git a/advanced.rst b/advanced.rst
index 0fe7806..6ade5b3 100644
--- a/advanced.rst
+++ b/advanced.rst
@@ -11,6 +11,7 @@
    :glob:
 
    advanced/connectivity
-   advanced/upf
+   advanced/p4-upf
+   advanced/bess-upf
    advanced/int
    advanced/qos