Stop using external links to IQP

Thanks to changes made in the internal app, we no longer need to use https://quantum.cloud.ibm.com. We can now use absolute links like / (homepage), /functions, etc. The web app will correctly understand this belongs to the Platform app and not our Docs app.

Fixing this has some benefits:

• Unblocks https://github.com/Qiskit/documentation/issues/3489 because it will now be easier to implement
• We can declare which IQP pages are expected and which are not. Our link checker will error if there are unexpected pages.

However, note that it's not 100% necessary. The web app will already convert https://quantum.cloud.ibm.com to https://eu-de.quantum.cloud.ibm.com for us.

Part 1: update our docs

Fix all instances of rg 'quantum.cloud.ibm.com' docs learning. Be very careful to only update links and not actual text. Don't update API docs

You will need to make sure that the link checker is passing. It probably won't naively. Run npm run check:internal-links. If there are failures, you will need to update this list so that our link checker thinks the files exist:

https://github.com/Qiskit/documentation/blob/e12ef6299015fd928cbf9bb370ebdd46232e8559/scripts/js/commands/checkInternalLinks.ts#L26

Part 2: update API docs

Rework relativizeLink in scripts/js/lib/api/updateLinks.ts, which is used to update our API docs to use the proper links for our app. It would only update Docs links before, but should probably update all links to https://quantum.cloud.ibm.com.