doc: number sections, add titles for modifiers

Author: ankostis

docs/source/index.rst

@@ -25,7 +25,8 @@ It might be of use in computer vision, machine learning and other data science d
 or become the core of a custom ETL pipelne.
 
 .. toctree::
-   :maxdepth: 2
+   :maxdepth: 3
+   :numbered:
 
    operations
    composition

docs/source/operations.rst

@@ -137,5 +137,11 @@ Modifiers on ``operation`` inputs and outputs
 
 Certain modifiers are available to apply to input or output values in ``needs`` and ``provides``, for example to designate an optional input.  These modifiers are available in the ``graphkit.modifiers`` module:
 
+
+Optionals
+^^^^^^^^^
 .. autoclass:: graphkit.modifiers.optional
+
+Sideffects
+^^^^^^^^^^
 .. autoclass:: graphkit.modifiers.sideffect

docs/source/reference.rst

@@ -2,6 +2,11 @@
 API Reference
 =============
 
+Package: `graphkit`
+===================
+
+.. automodule:: graphkit
+
 Module: `base`
 ==============