This includes documenting the new CSR approver built into the
controller manager and the kubelet alpha features for certificate
rotation.
Since the CSR approver changed over the 1.7 release cycle we need
to call out the migration steps for those using the alpha feature.
This document as a whole could probably use some updates, but the
main focus of this PR is just to get these features minimally
documented before the release.
Eric Chiang
fbb1904823