Improved i18n support

[chalin]

Carry-over items from #4443, and more:

• [i18n] Docs section weights need to be adjusted to work with all languages #4572
• [zh] Strip away embedded HTML en fragments #4602
• [i18n] Add language codes #4622
• [i18n] Support non-English banners with English fallback #4486
• [i18n & ja] Improved fallback config and normalize en doc links #4627
• [CI] i18n GH action check doesn't always work #4641
  • [CI & i18n] i18n script and doc improvements #4648
• [i18n] Propose way to associate translations to specific commit of default lang page #4582
• Ensure that search works as expected (a concern raised for other Docsy sites but that might apply here)
• [i18n] Support Netlify redirects for non en pages #4629
  • Ensure non-en community pages that are separate mounts or redirects are working
• Implement the registry as a redirect for non-en languages

Other, including ja and zh:

• [zh] English fallback-page canonical link should refer to en section page #4487
• Create docs-zh-approvers etc #4465
• [zh] faas/_index.md: fix invalid hash #4645
• Followups to Hugo docs edits: Clarify languageCode default in multilingual page gohugoio/hugoDocs#2564 and further contributions
• [i18n] Ensure repo link text is in the right language #4593
• Consider splitting up the Hugo config into parts?
• Reinstate Prettier to run over zh pages too - [CI & i18n] Support page formatting for ja and zh pages #4630
• Move images from under content/en/docs/demo/screenshots into static to ensure that they are shared(?)