Testing Python examples in the documentation

We can use the Python doctest module for testing examples in the documentation. Run it with

python -m doctest <filename>

on your markdown file.

See documentation contributions for how to write your examples such that they can be tested with doctest.

For instance, assume that you have the following example in your documentation

```python
    >>> 1 + 1  # doctest: +SKIP
    3

```

python -m doctest documentation_testing.md would print the following message:

**********************************************************************
File "documentation_testing.md", line 15, in documentation_testing.md
Failed example:
    1 + 1
Expected:
    3
Got:
    2
**********************************************************************

Now you copy the output following Got:, including the 4 indentation spaces, into your example.

Note that the comment #doctest: +SKIP was added to the above example in order to not trigger a doctest failure when validating the markdown file containing this guideline.