Refactor github workflows to add CI checks

[vicenteqa]

Description

This refactors the docs GitHub Actions workflows to provide safer and more consistent PR validation.

The motivation for this change is that merging Dependabot PRs in this repository was not safe enough: several dependency and GitHub Actions updates had little or no meaningful PR validation before being merged.

Changes included:

• Add reusable workflows for SUSE docs validation and docs site build validation.
• Add a new Docs CI workflow for pull requests affecting workflows, tool versions, docs content, the Antora site, or the UI bundle.
• Keep the existing publish workflow behavior, but make it call the reusable build workflow before deploying to GitHub Pages.
• Keep the SUSE docs validation workflow as a post-merge/manual wrapper and restrict its push trigger to main.
• Run npm ci, npm run format:check, extension tests, UI bundle build, and Antora site generation as part of PR build validation.
• Remove the standalone extension tests workflow because those tests now run through the shared docs site build validation.
• Update the CI and publishing overview documentation to describe the new workflow structure and behavior.

Fixes: N/A

How was this tested?

• Newlya added CI checks itself

Documentation changes

Yes

Updated content/trento-docs-site/ci-and-publishing-overview.adoc to describe the new PR CI workflow, reusable workflows, publishing behavior, SUSE docs validation behavior, and extension test coverage.

Additional information

The PR CI workflow does not publish or deploy anything. GitHub Pages deployment remains limited to the publish workflow triggers: push to main, scheduled runs, and manual runs.