Break up API docs by module.

docs/api.rst

 :mod:`persistent` API documentation
 ===================================
 
-Interfaces
-----------
+.. toctree::
+   :maxdepth: 2
 
-.. automodule:: persistent.interfaces
-
-  .. autointerface:: IPersistent
-     :members:
-     :member-order: bysource
-
-  .. autointerface:: IPersistentDataManager
-     :members:
-     :member-order: bysource
-
-  .. autointerface:: IPickleCache
-     :members:
-     :member-order: bysource
+   api/interfaces

docs/api/interfaces.rst

+:mod:`persistent.interfaces`
+===================================
+
+.. automodule:: persistent.interfaces
+
+  .. autointerface:: IPersistent
+     :members:
+     :member-order: bysource
+
+  .. autointerface:: IPersistentDataManager
+     :members:
+     :member-order: bysource
+
+  .. autointerface:: IPickleCache
+     :members:
+     :member-order: bysource
+