Add tutorial doctest module and enable top level await (#919)

Add a tutorial module with examples that can be tested with `doctest`.

In order to simplify the examples they can be run with doctest allowing
top level await statements by adding a fixture to patch the builtins
that xdoctest uses to test code.

---------

Co-authored-by: Teemu R. <tpr@iki.fi>

docs/source/conf.py

@@ -10,9 +10,10 @@
 # add these directories to sys.path here. If the directory is relative to the
 # documentation root, use os.path.abspath to make it absolute, like shown here.
 #
-# import os
-# import sys
-# sys.path.insert(0, os.path.abspath('.'))
+import os
+import sys
+
+sys.path.insert(0, os.path.abspath(".."))  # Will find modules in the docs parent
 
 
 # -- Project information -----------------------------------------------------