Add MR preview environments

As the title says, this MR aims to provide MR preview environments for easier reviews, with a particular emphasis on the fork-based workflows.

Note the following:

• The <root>/doc URL has been renamed to <root>/doc/latest for the "production" environment.
• The MR pipeline might need to be triggered by a reviewer/maintainer due to permissions and safety concerns.

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• 190fe717 - Draft: Implement multiple environments

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marked this merge request as ready

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merged