attempt to fix GH Pages publishing

[imjasonh]

This uses a GH Actions workflow to publish: https://github.com/marketplace/actions/deploy-mkdocs

It sets CUSTOM_DOMAIN: ko.build so there's a CNAME file at the root of the gh-pages branch, which is needed to serve requests from the custom domain.

This also fixes the edit URL to edit the main branch instead of the (default, 404) master branch.

[codecov-commenter]

Codecov Report

Merging #843 (a3a0789) into main (4f6bd9c) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #843   +/-   ##
=======================================
  Coverage   51.16%   51.16%           
=======================================
  Files          44       44           
  Lines        3385     3385           
=======================================
  Hits         1732     1732           
  Misses       1426     1426           
  Partials      227      227           

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