docs(ember): add deprecation guide for Comparable mixin

content/ember/v7/deprecate-comparable-mixin.md

@@ -0,0 +1,37 @@
+---
+title: Deprecate Comparable mixin
+until: 7.5.0
+since: 7.2.0
+---
+
+The `Comparable` mixin is deprecated. This mixin was private but as it may have been intimate API, per our policy we've added a deprecation that will remain for at least one LTS. 
+
+Apps and addons should stop importing or extending `Comparable`. To provide custom comparison behavior, define a function-valued `compare(other)` method directly on the object or class instead.
+
+Before:
+
+```js
+import EmberObject from '@ember/object';
+import Comparable from '@ember/-internals/runtime/lib/mixins/comparable';
+
+const Rectangle = EmberObject.extend(Comparable, {
+  compare(other) {
+    // custom comparison logic
+  },
+});
+```
+
+After:
+
+```js
+class Rectangle {
+  // Returns -1, 0, 1
+  compare(other) {
+    // custom comparison logic
+  },
+}
+```
+
+Defining `compare(other)` directly is sufficient. 
+
+However, if you need to `sort` a list of your objects, you will want to define a separate [`comparator function`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/sort#description).