docs: added index for deprecated files

docs/deprecated/index.txt

+======================================================
+Guides on backward compatibility & deprecated practice
+======================================================
+
+``Setuptools`` has undergone tremendous changes since its first debut. As its
+development continues to roll forward, many of the practice and mechanisms it
+had established are now considered deprecated. But they still remain relevant
+as a plethora of libraries continue to depend on them. Many people also find
+it necessary to equip themselves with the knowledge to better support backward
+compatibility. This guide aims to provide the essential information for such
+objectives.
+
+.. toctree::
+    :maxdepth: 1
+
+    python3
+    python_eggs
+    easy_install
+