Add zh_hant in opensource friday and rename zh to zh_hans

[jyangchisyan]

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Add zh_hant in opensource friday and rename zh to zh_hans

[tomthorogood]

closing/reopening to try and force codeql to run again

[AndreaGriffiths11]

Hey @jyangchisyan — thanks for opening #1322 and for the patience while this sat. The actual blocker turned out to be infrastructure, not your code: this repo's org-level code-scanning ruleset requires CodeQL results on every PR, but CodeQL default setup here only runs on pushes to main, not on fork PRs. So #1322 was permanently BLOCKED regardless of @tomthorogood's approval back in December.

To get it un-stuck I re-opened your change from a local branch in this repo as #1333 (you're preserved as the commit author). On top of your commit it picks up two small additions to address Copilot's review of the new PR:

redirect_from: /zh/ on the three zh_hans pages so existing inbound links (search results, external references) don't 404 after the rename.
Optional hreflang: field per locale + a tiny _includes/head.html change, so the alternate language links emit valid BCP 47 tags (zh-Hans / zh-Hant) instead of the underscore form. Every other locale is unchanged.
Closing this in favor of #1333. Really appreciate the contribution