Add docs on environments

[richfitz]

I've been meaning to write this for ages. There's still some details to add about parallel tasks, but that will open its own can of worms so leaving that for now

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (3d0597b) to head (4909e46).

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #152   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           26        26           
  Lines         2437      2437           
=========================================
  Hits          2437      2437           

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