[zh] Ensure zh fallback pages specify en page as canonical source

[chalin]

• Contributes to [zh] English fallback-page canonical link should refer to en section page #4487, well, it actually solves it, but I want to refactor the code. I'll do that later. I wanted to get this fix in before Google indexes the site :)

Test pages

Look at the page source to ensure that the canonical link is appropriately set:

• Preview: https://deploy-preview-4494--opentelemetry.netlify.app/

Before

$ curl -s https://opentelemetry.io/zh/docs/getting-started/ | perl -nle 'print $1 if /(canonical"?\s+href=.*?)>/i'
canonical href=https://opentelemetry.io/zh/docs/getting-started/

After

$ curl -s https://deploy-preview-4494--opentelemetry.netlify.app/zh/docs/getting-started/ | perl -nle 'print $1 if /(canonical"?\s+href=.*?)>/i'
canonical href=https://deploy-preview-4494--opentelemetry.netlify.app/docs/getting-started/