Support canonical URLs for Scaladoc

[armanbilge]

Many projects re-publish scaladocs for every published version of the project. Example:

• https://www.scala-lang.org/api/2.13.0/
• ...
• https://www.scala-lang.org/api/2.13.8/

This confuses search engines and "spreads" your hits across many (equally relevant) pages. The correct way to fix this from an SEO perspective is to include a "canonical" link for each page via an HTML tag. For example to https://www.scala-lang.org/api/current/...

Search engines view all versions of the page with the same canonical as effectively the same page. This allows them to "condense" all the hits and consistently deliver the canonical as the "definitive" URL for that page.

Further reading:

• https://haha.readthedocs.io/en/latest/canonical.html
• Google searches still lead to 0.18 docs http4s/http4s#5715 (comment)
• https://moz.com/blog/canonical-url-tag-the-most-important-advancement-in-seo-practices-since-sitemaps