docs: add missing site_url

[bearomorphism]

Relate #1716

Let's try if this fix the 404 page rendering issue.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.88%. Comparing base (120d514) to head (38d42e6).
⚠️ Report is 882 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1719      +/-   ##
==========================================
+ Coverage   97.33%   97.88%   +0.55%     
==========================================
  Files          42       60      +18     
  Lines        2104     2605     +501     
==========================================
+ Hits         2048     2550     +502     
+ Misses         56       55       -1     

Flag	Coverage Δ
unittests	97.88% <ø> (+0.55%)	⬆️

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