docs: add docsify site

[dnlup]

Fixes #232

• setup docsify
• setup publishing branch
• decide deployment strategy (SSR vs GH pages)
• setup ci

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (2d78dc9) to head (8699e1a).
⚠️ Report is 146 commits behind head on next.

Additional details and impacted files

@@            Coverage Diff            @@
##              next      #262   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           12        12           
  Lines          286       286           
=========================================
  Hits           286       286           

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[dnlup]

After some experimenting, it turns out that SSR is not well supported in docsify atm, leaving GH pages as the remaining option.

At least until docsifyjs/docsify#1227 is merged.