docs: add WeChat community invitation

[yzsnstotz]

Summary

Ports the content of #152 onto main. #152 was merged into feat/meridian-monorepo-portability (PR #139), which is still open and conflicting, so the change never reached main.

Cherry-picks both original commits (e80af3f3, 6b97cdf2) with authorship preserved:

• Add a compact WeChat community section to the repository homepage
• Place it after the Architecture section and before Quick start
• Present the invitation consistently across English, Simplified Chinese, and Japanese READMEs
• Preserve the original full-resolution QR image, displayed at 260px width
• Document the invitation expiry on August 21, 2026

Test plan

• Cherry-pick applied cleanly onto main (no conflicts)
• Diff vs main is exactly the 4 files / 89 insertions from docs: add WeChat community invitation #152
• git diff --check clean

Note on CI

The Typecheck, build, and test check is red on main itself and on every open PR, from a pre-existing failure in src/hub/router.test.ts (HubRouter run fallback does not reuse stale snapshot from before current run). It is unrelated to this docs-only change and is being fixed separately.