feat(mkdocs): add mkdocs

[FalcoSuessgott]

• add gotestsum
• bump to go 1.20
• add mkdocs
• validate goreleaser build in PR

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.75%. Comparing base (376d632) to head (1480670).
⚠️ Report is 21 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #92   +/-   ##
=======================================
  Coverage   93.75%   93.75%           
=======================================
  Files           5        5           
  Lines          64       64           
=======================================
  Hits           60       60           
  Misses          4        4           

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[FalcoSuessgott]

Fixes #91