Ensure code blocks in doc are doctests or @repl

[goerz]

This is currently on top of #41, so it will have to be rebased after #41 is either merged or closed.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.89%. Comparing base (1495e75) to head (3c62f3d).

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #42   +/-   ##
=======================================
  Coverage   85.89%   85.89%           
=======================================
  Files           1        1           
  Lines          78       78           
=======================================
  Hits           67       67           
  Misses         11       11           

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