Configure build environments for Analytics docs

Repurposing this ticket to make sure that:

1. The production, staging, and preview build environments all have Ruby and kramdown-asciidoc installed, so that the assembly.sh script will work in those environments.
2. The Antora Collector extension is enabled for the Antora playbook in the production and staging build environments.

Any changes which are necessary to update our Antora build to fetch markdown pages from the AsterixDB mirror. For example, configuring Antora collector, if necessary.

The AsterixDB mirror uses the same development branches as cbas-core, and we should use those branches to find the correct versions of the upstream markdown files. See Server Code Names and Version Numbers for details.

Note that we only copy the markdown files from the AsterixDB mirror. We do not copy their image files or example files. We always use our own local image files and example files, even for material which has been transcluded from the AsterixDB mirror. This means that, if any of the documentation from the AsterixDB mirror contains a link to an image or an example file, we need to make sure we've stored an image or example file with the same name in the place where the converted AsterixDB documentation expects to find it.