Synchronize the fixes from #1936, #1824, and #1961 across translated editions and normalize C# and Dart formatting. Move Java CI to Ubuntu 24.04 so its Java 11/17 checks can run.
Add dedicated chapter exercise sections and generated exercise pages for the English, Japanese, and Russian editions.
Add navigation for all published languages while keeping Taiwan Traditional Chinese exercise pages generated at build time.
* docs: add Japanese documents (`ja/docs`)
* docs: add Japanese documents (`ja/codes`)
* docs: add Japanese documents
* Remove pythontutor blocks in ja/
* Add an empty at the end of each markdown file.
* Add the missing figures (use the English version temporarily).
* Add index.md for Japanese version.
* Add index.html for Japanese version.
* Add missing index.assets
* Fix backtracking_algorithm.md for Japanese version.
* Add avatar_eltociear.jpg. Fix image links on the Japanese landing page.
* Add the Japanese banner.
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Co-authored-by: krahets <[email protected]>
Yudong Jin