docs/: add sub document data-model.rst

Currently just a stub (but with a hereby fixed anchor) to allow linking to it
from other part of the development documentation. Will be fillen later on with
an abstract description of the Software Heritage data model.

docs/data-model.rst

+.. _data-model:
+
+Software Heritage data model
+============================
+
+TODO
+
+Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor
+incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis
+nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo
+consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse
+cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non
+proident, sunt in culpa qui officia deserunt mollit anim id est laborum.

docs/index.rst

@@ -6,6 +6,11 @@ Software Heritage - Development Documentation
    :caption: Contents:
 
 
+Overview
+--------
+
+* :ref:`data-model`
+
 
 Indices and tables
 ==================