docs: Add inheritance diagram to exceptions documentation

In light of exception hierarchy adjustments currently in progress
(I0e1e81b513fbbc7adb8bfaa1244993e345ab70d3) it becomes really helpful to
have inheritance diagram for exceptions in documentation.

Change-Id: Ie8f7b160de08fa8a91499cac2f94a76aff186e67

doc/source/conf.py

@@ -11,6 +11,7 @@ sys.path.insert(0, os.path.abspath('../..'))
 extensions = [
     'sphinx.ext.autodoc',
     'sphinx.ext.doctest',
+    'sphinx.ext.inheritance_diagram',
     'sphinx.ext.intersphinx',
     'sphinx.ext.viewcode',
     'oslosphinx'

doc/source/exceptions.rst

@@ -2,4 +2,8 @@
 Exceptions
 ----------
 
+.. inheritance-diagram::
+    taskflow.exceptions
+    :parts: 1
+
 .. automodule:: taskflow.exceptions