build(aio): create minLength content rule (#22759)

This rule can be used to ensure that properties contain a minimum
number of characters.

PR Close #22759

aio/tools/transforms/angular-api-package/content-rules/minLength.js

@@ -0,0 +1,8 @@
+module.exports = function createMinLengthRule(minLength) {
+  minLength = minLength || 2;
+  return (doc, prop, value) => {
+    if (value.length < minLength) {
+      return `Invalid "${prop}" property: "${value}". It must have at least ${minLength} characters.`;
+    }
+  };
+};