Add feature gate for MutatingAdmissionPolicy in v1.30

Author: tengqm

content/en/docs/reference/command-line-tools-reference/feature-gates/mutating-admission-policy.md

@@ -0,0 +1,16 @@
+---
+title: MutatingAdmissionPolicy
+content_type: feature_gate
+_build:
+  list: never
+  render: false
+
+stages:
+  - stage: alpha 
+    defaultValue: false
+    fromVersion: "1.30"
+---
+In Kubernetes {{< skew currentVersion >}}, this feature gate has no effect.
+A future release of Kubernetes may use this feature gate to enable
+the MutatingAdmissionPolicy in admission chain.
+