Docs: Move API reference to a separate page

docs/api.rst

+API reference
+=========================
+
+.. toctree::
+   :maxdepth: 1
+   :caption: Contents
+
+   parser
+   transformer
+   operator
+   exceptions
+

docs/exceptions.rst

+ransack.exceptions module
+================================================================================
+
+.. automodule:: ransack.exceptions
+   :members:
+   :undoc-members:
+   :show-inheritance:

docs/index.rst

@@ -8,7 +8,5 @@ Python library for filtering, querying or inspecting almost arbitrary data struc
    :maxdepth: 1
    :caption: Contents:
 
-   parser
-   transformer
-   operator
+   api