Link checker should ban external links to IQP

It's come up a few times in content review that contributors don't realise they need to remove the /en/ from links they put in content. If technically possible would be great to include a CI check that looks for these so the docs content writers don't have to keep cleaning them up after they've merged

Update from Eric on July 10, 2025: Better is to ban absolute links entirely to docs and learning. We should be using relative links like /docs/my-page.

How to fix

1. The first thing we need to do is make sure we're recognizing links that start with https://quantum.cloud.ibm.com as internal links. We should do that by adding a new test in scripts/js/lib/links/extractLinks.test.ts.

   I'd recommend copying the following test, and editing the markdown string to include some links starting with https://quantum.cloud.ibm.com. Then expect internalLinks to include these links.
   https://github.com/Qiskit/documentation/blob/de77a8ed372d4e0aeb99928e365fa6029ce455da/scripts/js/lib/links/extractLinks.test.ts#L85-L99

2. With the tests in place, edit parseLinks() to recognise links starting with https://quantum.cloud.ibm.com as internal links:

   https://github.com/Qiskit/documentation/blob/de77a8ed372d4e0aeb99928e365fa6029ce455da/scripts/js/lib/links/extractLinks.ts#L63-L97

3. Finally, we need to adapt InternalLink to handle absolute internal links.

   First, the constructor will need to accept links starting with https://quantum.cloud.ibm.com.
   https://github.com/Qiskit/documentation/blob/main/scripts/js/lib/links/InternalLink.ts#L44-L49

   Then, we'll need to return an appropriate error message from InternalLink.check() if the link is absolute:

   https://github.com/Qiskit/documentation/blob/main/scripts/js/lib/links/InternalLink.ts#L177-L196

   It'd be good to add some test cases to scripts/js/lib/links/InternalLink.test.ts

Start with the tests in scripts/js/lib/links/extractLinks.test.ts and scripts/js/lib/links/InternalLink.test.ts, then read parseLinks() in scripts/js/lib/links/extractLinks.ts. Update the link handling in scripts/js/lib/links/InternalLink.ts so absolute quantum.cloud.ibm.com links are recognized and produce the appropriate error. Done means the new absolute-link cases pass in both test files.