Remove migration jobs in preparation for launch

Remove migration scripts in preparation for launch.

Just drop the CI jobs for now; we can remove the actual code later.

This work includes copying over navigation.yaml and redirects.yaml from gitlab-docs. Note that navigation.yaml will need to be updated to reflect its new format. An easy way to do this would be to grab the latest artifact and copy navigation.yaml from there.

This will need to be merged after we update the actual content as part of launch, but we can create the MR in advance.