Duplicate complete checking content

[mattmassicotte]

Was here: https://www.swift.org/documentation/concurrency/

And now is here: https://www.swift.org/migration/documentation/swift-6-concurrency-migration-guide/completechecking

cc @shahmishal @hborla

[hborla]

I think we should redirect https://www.swift.org/documentation/concurrency/ to https://www.swift.org/migration/documentation/swift-6-concurrency-migration-guide/completechecking or just to Swift.org/migration