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/bess-upf.rst b/advanced/bess-upf.rst
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+.. SPDX-FileCopyrightText: 2021 Open Networking Foundation <info@opennetworking.org>
+.. SPDX-License-Identifier: Apache-2.0
+
+BESS-based User Plane Function (BESS-UPF)
+=========================================
+
+.. note::
+
+    We are transitioning BESS-UPF from SD-Core to SD-Fabric.
+    We are also working on consolidating interfaces required by BESS-UPF to simplify deployment.
+    More documentation and architectural contents are coming shortly once these are done.
+    For now, please refer to :ref:`BESS UPF Deployment <aether:edge_deployment/bess_upf_deployment:bess upf deployment>`
+    to learn more about how to configure and deploy BESS-UPF.
diff --git a/advanced/upf.rst b/advanced/p4-upf.rst
similarity index 100%
rename from advanced/upf.rst
rename to advanced/p4-upf.rst