docs: keep only bilingual (English + Simplified Chinese) README and CONTRIBUTING

Remove ja/ko/ru/zh-TW translations of README.md and CONTRIBUTING.md.
The tracker workflow now only syncs docs/README.zh-CN.md; language links
in README.md, CONTRIBUTING.md, and the AGENTS.md layout are updated to
the two maintained languages.

Co-Authored-By: Claude <noreply@anthropic.com>

.github/workflows/gitea-tracker.yml

@@ -101,7 +101,7 @@ jobs:
         if: steps.version.outputs.skip != 'true'
         run: |
           NEW="${{ steps.version.outputs.new_version }}"
-          for f in docs/README.{ja,ko,ru,zh-CN,zh-TW}.md; do
+          for f in docs/README.zh-CN.md; do
             sed -i "/TRACKER:LATEST-TESTED/s/Gitea [0-9.]*/Gitea ${NEW}/" "$f"
           done