[Ops-Impr] Need to get builddocs generative tests, such as Sphinx PDF generation, into GitHub Actions

Description

This is a follow-up to #59446

• Which was a PR made because of an issue discovered downstream with our builddocs repo on GitLab.

Right now, PDF-breaking changes (or EPUB-breaking changes) won't be caught until the pipelines in builddocs launch against the latest updates to the master branch of this repo.

Suggested Fix

Add another docs test, similar to how we test for building the man pages and HTML. We should, at a minimum, include PDF building tests. EPUB builds seem to be more resilient, and would be an optional add. Especially because I don't know what the long-term value of maintaining the EPUB is.

• Dockerfile reference: https://gitlab.com/saltstack/open/docs/builddocs/-/blob/master/container/Dockerfile
• Build job reference: https://gitlab.com/saltstack/open/docs/builddocs/-/blob/master/.gitlab-ci.yml#L53-65
  • build-pdf.sh script referenced: https://gitlab.com/saltstack/open/docs/builddocs/-/blob/master/build_pdf.sh
  • build-env.sh script referenced: https://gitlab.com/saltstack/open/docs/builddocs/-/blob/master/build_env.sh

Since builddocs is a public repo, we'd be able to simply git clone it into an action, and run the necessary commands from there.