docs: add Russian translation and update navigation in READMEs

[rastuhacode]

This PR

1. Adds README_ru.md with Russian translation of original README.md.
2. Updates links in all READMEs by adding Russian redirect option.
3. Updates the navigations with actual second-level headers (##) in al READMEs.

From discussion #130

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.87%. Comparing base (e42ef8e) to head (7bdac4c).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #133   +/-   ##
=======================================
  Coverage   92.87%   92.87%           
=======================================
  Files          28       28           
  Lines         449      449           
  Branches      105      110    +5     
=======================================
  Hits          417      417           
  Misses         32       32           

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Project	Deployment	Actions	Updated (UTC)
capsule-render	Ready	Preview, Comment	Jun 9, 2026 11:49am

[kyechan99]

lgtm