CI: add link-checking GH action, stop failing Netlify build on link-check issues

[chalin]

• Contributes to Make build and CI more contributor friendly #2448
• Adds a link-checking GH action
• Changes Netlify build: link checking is still done, but the build won't fail if an issue is reported by the link checker

This change has been tested with:

• No link issues
• htmltest link issues
• Refcache update required

In all cases, the Netlify build passes and, when appropriate the "Check links" GH action reports problems.