Add more Korean translations

[jslee02]

Expand the Korean ReadTheDocs coverage:

• finish the remaining sections of the multi-pendulum tutorial (Lessons 1–3) so Korean readers can complete the walkthrough without dropping back to English
• translate the "Who Uses DART?" overview plus supporting installation/gallery snippets so the ko site highlights real-world usage
• keep the PO files tidy by filling previously empty msgstr blocks and leaving shared English where appropriate

pixi run lint (with CMAKE_BUILD_PARALLEL_LEVEL=12) was executed to format and verify the tree before committing. No additional tests were required because this is translation-only.

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Before creating a pull request

• Run pixi run test-all to lint, build, and test your changes
• Add unit tests for new functionality
• Document new methods and classes
• Add Python bindings (dartpy) if applicable

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 62.27%. Comparing base (37a67ac) to head (f0645e7).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2171   +/-   ##
=======================================
  Coverage   62.27%   62.27%           
=======================================
  Files         393      393           
  Lines       33207    33207           
  Branches     4138     4138           
=======================================
  Hits        20679    20679           
  Misses      12528    12528           

Flag	Coverage Δ
unittests	62.27% <ø> (ø)

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