[CI] Ensure that every page has a canonical link

[chalin]

• Prep for [zh] English fallback-page canonical link should refer to en section page #4487
• Ensures that every page has a canonical URL, as seems to be recommended by best practices

Test pages

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

• https://deploy-preview-4492--opentelemetry.netlify.app/

All pages have canonical link:

$ curl -s https://opentelemetry.io/docs/ | perl -nle 'print $& if /canonical"?\s+href=.*?>/i'
canonical href=https://opentelemetry.io/docs/>

The canonical_url param takes precedence:

$ curl -s https://opentelemetry.io/blog/2019/opentelemetry-governance-committee-explained/ | perl -nle 'print $& if /canonical"?\s+href=.*?>/i'
canonical href=https://medium.com/opentelemetry/opentelemetry-governance-committee-explained-860353baba0>

But zh pages need fixing still (it should refer to en page), this will be addressed via #4494:

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