Add docc documentation

[alexdeem]

• add dependency on swift-docc-plugin
• Add docc documentation comments
• Have swiftlint complain about missing documentation
• Update Copyright Headers

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.19%. Comparing base (5640eae) to head (8f853a8).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop      #68   +/-   ##
========================================
  Coverage    99.19%   99.19%           
========================================
  Files            8        8           
  Lines          371      371           
========================================
  Hits           368      368           
  Misses           3        3           

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