[i18n] Initial multilingual support for redirects with ja example

[chalin]

• Contributes to:
  • [i18n] Support Netlify redirects for non en pages #4629
  • Add Japanese version of website pages #4592
• Adds partials in support of multilingual support for redirects
• Adds redirect rules for /en and /en/*
• Adds /ja/docs/specs/_index.md as a redirect placeholder page

Preview / redirect test:

• https://deploy-preview-4687--opentelemetry.netlify.app/ja/docs/specs/ - should redirect to en page

Screenshot

[chalin]

All green, merging.