docs: link pages together from a TOC

docs/data-model.rst

-:orphan:
-
 .. _data-model:
 
 Data model

docs/index.rst

@@ -5,18 +5,10 @@ Software Heritage - Data model
 
 Implementation of the :ref:`data-model` to archive source code artifacts.
 
-
-Overview
---------
-
-* :ref:`data-model`
-* :ref:`persistent-identifiers`
-
-
-Reference Documentation
------------------------
-
 .. toctree::
-   :maxdepth: 2
+   :caption: Overview:
+   :titlesonly:
 
+   data-model
+   persistent-identifiers
    /apidoc/swh.model

docs/persistent-identifiers.rst

-:orphan:
-
 .. _persistent-identifiers:
 
+======================
 Persistent identifiers
 ======================
 
+Description
+===========
+
 You can point to objects present in the Software Heritage archive by the means
 of **persistent identifiers** that are guaranteed to remain stable (persistent)
 over time. Their syntax, meaning, and usage is described below. Note that they