This PR fixes several things in the admission-controllers page:
- The `PodSecurity` plugin is enabled by default, but it was not listed so;
- The `apiserver.config.k8s.io/v1alpha1` has been deprecated since v1.17, we are still documenting it side by side with the `apiserver.config.k8s.io/v1` API group;
- The `eventratelimit.admission.k8s.io/v1alpha1` API could use a better reference rather than the design doc; **The imagepolicy.v1alpha1 API is not documented anywhere, I'll add it later on.**
- There are statements about future, which should be removed;
- We are supposed refer to the `LimitRage` API reference rather than pointing users to the design docs;
- We are supposed refer to the `ResourceQuota` API reference rather than pointing users to the design docs;
- There are long lines in the page source which could have been wrapped properly.
Qiming Teng