doc: reorg part 7 - add package/index.rst file

This file should work as-is, but will only say "(No package docs are
installed.)".  gendoc will replace that with actual package docs.

documentation/source/package/index.rst

@@ -0,0 +1,22 @@
+******************
+Dylan Package Docs
+******************
+
+Documentation for packages listed in the Dylan package catalog.
+
+.. note:: The documentation for some packages is still part of the Open Dylan
+   project, available in the `Library Reference </library-reference/>`_.  Over
+   time more of those docs will be hosted here as they're moved into their own
+   repositories.
+
+.. Text between the markers is replaced by gendoc.
+
+.. begin-body
+
+(No package docs are installed.)
+
+.. end-body
+
+.. begin-toctree
+
+.. end-toctree