check PR branches for broken links

[qrkourier]

Now that we have broken link detection for the main branch we will try to detect broken links earlier by targeting PR branches' Vercel preview URLs with the link checker scripts.

The new PR checks will:

• be implemented in GH Actions
• trigger when a PR is created or updated
• scrape the Vercel preview URL from the GitHub API
• run the crawler and the external/popular links checker scripts against the preview URL

Repo admins can then create branch protection rules determining how a check failure is handled, e.g., whether or not a failure blocks merge to main.