Add support for canonical URLs

[bmorelli25]

Summary

We need to add the ability to add canonical URLs to the v3 doc build:

• current should always be the source of truth.
• previous versions should automatically set the current version of a page, if it exists, as canonical
• writers should be able to override the default in the frontmatter of a page

From @KOTungseth

I just met with our website team (Leah and Casey) to discuss canonical URLs. For context, one of the recommendations that came out of the Botify audit was to implement canonical tags, which will help to consolidate page authority.
Back in 2022, Greg implemented canonicals in our Asciidoc system (elastic/docs#2357). We do have canonicals on some areas of the Docs (https://github.com/elastic/apm-server/edit/7.15/docs/guide/overview.asciidoc), but the Asciidoc build no longer picks them up, so canonicals do not exist according to the HTML output.