Refactor layouts/index.redirects

[chalin]

• Contributes to Redirect layout rework and fixes #4899
• In prep for:
  • [i18n] Support Netlify redirects for non en pages #4629
  • [i18n] Ensure that fall-back redirect pages like API and Examples under Language work for non-en localizations #4898

The generated _redirects file is the same before and after this PR:

$ npm run build                          
...
$ (cd public && git diff -bw) | grep ^diff
diff --git a/site/index.html b/site/index.html

Sanity tests or redirect rules

• https://deploy-preview-4900--opentelemetry.netlify.app/vendors
• https://deploy-preview-4900--opentelemetry.netlify.app/en/docs
• https://deploy-preview-4900--opentelemetry.netlify.app/ja/docs/specs/status