Google pics up old tutorials, try to fix that

Add canonical links in the 5.x version of the docs tutorials pointing at the new version. The "see latest version" link breaks there because the URL has changed.

The canonical link will hopefully get the search engines to change their index.

This commit allows us to set arbitrary canonical header links per page: https://github.com/citusdata/citus_docs/pull/424/commits/e3b46cc63d2eb4d65ecc2b4d8402fed045c1e178

Cherry-pick it to the previous version of docs and modify it to affect just the tutorials.

(Thanks to @lithp for reporting the SEO problem)