[infra] Adjust site config etc in preparation for multilingual support

[chalin]

Context:

• Internationalization (i18n) and localization (l10n) #4302
• Add Chinese version of website #2402
  • [zh] Add zh version of homepage #4341

Tasks

• Clarify purpose of multilingual site title gohugoio/hugoDocs#2563
• [infra] Fix spec markdown link processing #4445
• [infra] .prettierignore all languages other than en for now #4447
• Upgrade to Hugo 0.125.7, Docsy at HEAD #4456
• Clean up site param in prep for multilang #4459
• [i18n] Base config for zh #4462
• Fix duplicate redirects file issue - [i18n] Don't generate _redirects for non-en languages #4466

Carry forward:

• Clarify languageCode default in multilingual page gohugoio/hugoDocs#2564
  Describe .Language.LanguageCode fallback value gohugoio/hugoDocs#2574
• Ensure that search works as expected (a concern raised for other Docsy sites but that might apply here)

[chalin]

This is proving more difficult than I would have hoped. Maybe this old thread might help:

• https://discourse.gohugo.io/t/content-mount-problem-for-single-language-multilingual-site/37215

Consider added each language content dir via a mount point rather than use contentDir?

[chalin]

All done. For items carried forward, see:

• More i18n support, some zh-specific #4467