Created opendc-demos folder and added the first demo

[DanteNiewenhuis]

Summary

Created opendc-demos folder and added the first demo.

It also adds demo_converter.py, which can convert a demo folder to markdown.
This makes it much easier to turn a demo into a tutorial for the website.

Implementation Notes ⚒️

N / A

External Dependencies 🍀

N / A

Breaking API Changes ⚠️

N / A

Simply specify none (N/A) if not applicable.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 61.40%. Comparing base (c7e303a) to head (4b14734).

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #340   +/-   ##
=========================================
  Coverage     61.40%   61.40%           
  Complexity       50       50           
=========================================
  Files           339      339           
  Lines         11159    11159           
  Branches       1104     1104           
=========================================
  Hits           6852     6852           
  Misses         3917     3917           
  Partials        390      390           

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