Add release notes to read-the-docs.

doc/index.rst

@@ -17,6 +17,7 @@ Contents:
 
    userguide/index.rst
    tutorials/index.rst
+   release/index.rst
 
 
 ##################

doc/release/index.rst

+=============
+Release notes
+=============
+
+.. toctree::
+   :maxdepth: 2
+
+   smtk-21.07.rst
+   smtk-21.05.rst

doc/release/smtk-21.05.rst

+.. _release-notes-21.05:
 
 ========================
 SMTK 21.05 Release Notes

doc/release/smtk-21.07.rst

+.. _release-notes-21.07:
+
 =========================
 SMTK 21.07 Release Notes
 =========================
 
-See also `SMTK 21.05 Release Notes`_ for previous changes.
-
-.. _`SMTK 21.05 Release Notes`: smtk-21.05.md
+See also :ref:`release-notes-21.05` for previous changes.
 
 Registration Changes
 ====================