fix(doc): don't briefly show 404 when navigating

[vegerot]

Screen.Recording.2025-05-09.at.12.20.18.PM.mov

Summary:
Currently, clicking on links will briefly show a 404 page before being
redirected. This is because we incorrectly set the default route to be zh
while the default language is en. This commit fixes the issue by aligning the
default route and language.

Test Plan:

1. Go to homepage
2. Click "Introduction"
   Before: Briefly see 404 before being redirected
   After: See the correct page

[changeset-bot]

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Latest commit: e8de2dc

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[caohuilin]

Thanks!