[doc] Use Google-hosted PT Sans for Russian docs

[aobolensk]

• import PT Sans from Google Fonts in custom CSS for /ru docs
• keep Sphinx setup that loads this stylesheet

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.73%. Comparing base (756bc23) to head (c1a0a1e).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #494   +/-   ##
=======================================
  Coverage   91.73%   91.73%           
=======================================
  Files          11       11           
  Lines         363      363           
  Branches      131      131           
=======================================
  Hits          333      333           
  Misses         10       10           
  Partials       20       20           

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[github-actions]

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