docs(readme): expand localized readmes

[jyxjjj]

Motivation

• Consolidate non-English documentation into a single README/ directory to make localized README files easier to find and maintain.
• Provide new localized README variants for additional audiences by adding Korean, German, Russian, French, Spanish, and Arabic translations.
• Update repository navigation and any hard-coded sample URLs to reference the new paths so links remain valid after the move.

Description

• Moved existing localized files into README/ (e.g. README_cn.md -> README/README_cn.md, README_ja.md -> README/README_ja.md, README_nl.md -> README/README_nl.md) and updated their internal root-relative links to point to ../ where appropriate.
• Added six new localized README files: README/README_kr.md, README/README_de.md, README/README_ru.md, README/README_fr.md, README/README_es.md, and README/README_ar.md containing localized content and a consistent navigation block.
• Updated the root README.md language navigation to link to the new README/ paths for non-English documents (for example ./README/README_cn.md).
• Updated the drivers/url_tree/meta.go UrlStructure default sample to reference README/README_cn.md and formatted the modified file with gofmt.

Testing

• Ran a local README link validation script (python3 - <<'PY' ...) to verify local relative links and it succeeded.
• Ran unit tests for the updated driver package with go test ./drivers/url_tree and they passed.

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Codex Task