[CI]: Standardize the PR preview-deploy workflow across meshery-academy repos

Current Behavior

The preview-deploy workflow has drifted from how the rest of the pipeline is built — it installs its own toolchain and builds the site directly instead of going through the standard make targets that production already uses. Additionally, it relies on checkouts of pull request head/merge refs under pull_request_target, which GitHub is deprecating for security reasons.

Desired Behavior

The preview-deploy workflow needs to be brought in line with GitHub's recommended patterns for handling pull request previews, and that change should be applied consistently across every repository subscribed to the meshery-academy topic so the workflow stays uniform wherever it's used.

Implementation

Split the workflow into a build stage that runs under pull_request using the shared make targets, handing its output to a separate privileged deploy stage via an artifact — the pattern GitHub documents for this use case. Roll the updated workflow out to all meshery-academy-subscribed repositories.

Acceptance Tests

• The workflow successfully builds the preview via make targets.
• The workflow successfully passes the built artifacts to the deployment stage via GitHub artifacts.
• The workflow no longer relies on checking out PR refs under pull_request_target.

Contributor Guide and Resources

• 📚 Instructions for contributing to academy-example
  • academy-template documentation site, example and theme
• 🎨 Wireframes and designs for Layer5 site in Figma (open invite)
• 🙋🏾🙋🏼 Questions: Layer5 Discussion Forum and Layer5 Community Slack

Start with the existing preview-deploy workflow in academy-example and inspect the repository's shared make targets. Compare the current pull_request_target flow with GitHub's documented build-and-deploy artifact pattern, then identify every repository subscribed to the meshery-academy topic. Done means all subscribed repositories use the same workflow, build through make, pass artifacts between stages, and no longer check out PR refs under pull_request_target.