Add Mixin deprecation guide

content/ember/v6/deprecate-ember-mixin.md

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+---
+title: Deprecating Ember.Mixin
+until: 7.0.0
+since: 6.5.0
+---
+
+`Ember.Mixin` is deprecated. This is part of the legacy `@ember/object` API and is not compatible with native classes. Instead of using mixins, you should refactor your code to use class-based patterns.
+
+One common pattern to replace mixins is to use class decorators.
+
+### Before: Using Mixins
+
+Here is an example of a mixin that provides "editable" functionality to a class:
+
+```javascript
+// app/mixins/editable.js
+import Mixin from '@ember/object/mixin';
+
+export default Mixin.create({
+  isEditing: false,
+
+  edit() {
+    console.log('starting to edit');
+    this.set('isEditing', true);
+  },
+});
+```
+
+```javascript
+// app/models/comment.js
+import EmberObject from '@ember/object';
+import EditableMixin from '../mixins/editable';
+
+export default class Comment extends EmberObject.extend(EditableMixin) {
+  post = null;
+}
+```
+
+### After: Using Class Decorators
+
+You can achieve the same result by creating a class decorator. This decorator will add the same properties and methods to the class it's applied to.
+
+First, create the decorator function:
+
+```javascript
+// app/decorators/editable.js
+import { tracked } from '@glimmer/tracking';
+
+export function editable(klass) {
+  return class extends klass {
+    @tracked isEditing = false;
+
+    edit() {
+      console.log('starting to edit');
+      this.isEditing = true;
+    }
+  };
+}
+```
+
+Then, apply the decorator to your class:
+
+```javascript
+// app/models/comment.js
+import EmberObject from '@ember/object';
+import { editable } from '../decorators/editable';
+
+@editable
+export default class Comment extends EmberObject {
+  post = null;
+}
+```
+
+This approach provides the same functionality as the mixin but uses standard JavaScript features, making the code easier to understand.