Automate the guidebook release process

[semioticrobotic]

In #266, @sanchuanhehe outlined the feasibility of automating the guidebook release process:

I think we are very close—the main building blocks are already in place. HTML, PDF, and EPUB are built and validated automatically, and the Pages deployment is working.

What remains is mostly the final orchestration: trigger the production workflow from a guidebook release tag, build from that exact tag, publish the downloadable artifacts, deploy the website under /production/<tag>/, and update /production/ to the latest stable release.

I would consider this a relatively small additional investment: probably one focused follow-up change and validation during the first automated release. The only part that may require owner or admin involvement is authorizing the cross-repository trigger.

This issue tracks discussion and implementation of release automation solutions.