Add order model example to docs

[pbadeer]

Currently the "Checkout and Payment" documentation page has an example payment.py that does not work if you follow the guide, because the guide never has you set up the custom Order model that it's using behind the scenes.

Thank you for contributing to Salesman, before you continue make sure that:

• Tests still pass: poetry run pytest
• There are no lint errors: poetry run pre-commit run --all-files

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (8105e84) 98.17% compared to head (32a3e3b) 98.17%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #37   +/-   ##
=======================================
  Coverage   98.17%   98.17%           
=======================================
  Files          49       49           
  Lines        2032     2032           
=======================================
  Hits         1995     1995           
  Misses         37       37           

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[pbadeer]

I saw where the orders.py file was being surfaced in the docs and I've corrected my documentation changes to reference your existing pattern of using swappable models.