Adds docs/th/README.md with a concise onboarding-style Thai
translation mirroring the docs/vi-VN format. Updates the language
switchers in the English, Simplified Chinese, Traditional Chinese,
Japanese, Korean, Portuguese (BR), Russian, Turkish, Vietnamese,
and Simplified Chinese docs READMEs to link to the new Thai page.
The English README remains the canonical source of truth; the Thai
page links back to it for full content.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
- add a current Vietnamese onboarding README adapted from stale community PR #1322
- link Vietnamese from the existing localized README language selectors
- keep stale full translation content out of tree while preserving useful contributor work
