Right now, we are using the extension sphinx_redirects to redirect the old Aer docs to the new style. (Note that this before the Ecosystem migration):

But sphinx_redirects does not properly set its Sphinx config if it can be parallelized. So it causes -j auto to fail, which makes the docs build much slower. That will be an issue when we migrate the metapackage docs to Terra.

Instead, Matthew T. had the simpler idea to move these redirects into the Cloudflare config for qiskit.org. That is already how we did the ecosystem/ redirects.