feat: support fully-qualified absolute canonical URLs

[pepopowitz]

Description

Part of #3731.

Ports d00a54d, which was required during the 8.1 archival, to the main docs. This allows us to specify canonical URLs which include https://docs.camunda.io, which is necessary when archiving a version to https://unsupported.docs.camunda.io.

When should this change go live?

• This is a bug fix, security concern, or something that needs urgent release support.
• This is already available but undocumented and should be released within a week.
• This on a specific schedule and the assignee will coordinate a release with the DevEx team. (apply hold label or convert to draft PR)
• This is part of a scheduled alpha or minor. (apply alpha or minor label)
• There is no urgency with this change and can be released at any time.

PR Checklist

• My changes are for an already released minor and are in /versioned_docs directory.
• My changes are for the next minor and are in /docs directory (aka /next/).

• My changes require an Engineering review, and I've assigned the Engineering DRI or delegate.
• My changes require a technical writer review, and I've assigned @christinaausley as a reviewer.

• My changes require a docs infrastructure review.