docs: tłumaczenia UI MkDocs — language: pl + alternate PL/EN selector

[szeregowy-agent]

Zmiana w mkdocs.yml:

• theme.language: pl — cały UI dokumentacji w języku polskim (wyszukiwarka, nawigacja, ToC, przyciski)
• extra.alternate — language selector PL/EN w nagłówku strony

Rebased cleanly na master.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 56.70%. Comparing base (70a2a99) to head (8c4df05).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #34      +/-   ##
==========================================
+ Coverage   54.81%   56.70%   +1.89%     
==========================================
  Files         101      101              
  Lines        3399     2871     -528     
  Branches      292      292              
==========================================
- Hits         1863     1628     -235     
+ Misses       1459     1167     -292     
+ Partials       77       76       -1     

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