Adding link checking to the CI will cut down on broken links.

I've found a tool to accomplish this, which can be seen in this PR in my fork:

• PR: https://github.com/dhaiducek/rhacm-docs/pull/1
• Workflow: https://github.com/dhaiducek/rhacm-docs/actions/runs/8926133440/job/24516467295?pr=1
• Summary: https://github.com/dhaiducek/rhacm-docs/actions/runs/8926133440?pr=1#summary-24516467295

In the POC, I set it to only perform checks for external links (since I think the other check should validate internal links?)