docs: reduced top index to three entries

The top index now points to three files: setuptools.txt (user guide),
development.txt (development guide) and a WIP index file that is to
summarized deprecated practice that remains relevant for backward
compatibility.

docs/index.txt

@@ -16,10 +16,6 @@ Documentation content:
 .. toctree::
    :maxdepth: 2
 
-   setuptools
-   pkg_resources
-   python3
-   development
-   roadmap
-   Deprecated: Easy Install <easy_install>
-   history
+   User guide <setuptools>
+   Development guide <development>
+   Backward compatibility & deprecated practice <deprecated/index>