Document removal of $children API on Migration guide (#670)

* feat: added children to migration guide

* feat: added link to migration introduction

Author: NataliaTepluhina

src/guide/migration/children.md

@@ -0,0 +1,38 @@
+---
+badges:
+  - removed
+---
+
+# $children <MigrationBadges :badges="$frontmatter.badges" />
+
+## Overview
+
+`$children` instance property removed from Vue 3.0 and no longer supported.
+
+## 2.x Syntax
+
+In 2.x, developers could access direct child components of the current instance with `this.$children`:
+
+```html
+<div ref="app" id="app">
+  <img alt="Vue logo" src="./assets/logo.png" width="25%" />
+  <my-button>Change logo</my-button>
+</div>
+```
+
+```js
+export default {
+  name: "App",
+  components: {
+    MyButton,
+  },
+  mounted() {
+    console.log(this.$children); // [VueComponent]
+  },
+};
+</script>
+```
+
+## 3.x Update
+
+In 3.x, `$children` property is removed and no longer supported. Instead, if you need to access a child component instance, we recommend using [$refs](/guide/component-template-refs.html#template-refs).

src/guide/migration/introduction.md

@@ -109,9 +109,10 @@ The following consists a list of breaking changes from 2.x:
 ### Removed APIs
 
 - [`keyCode` support as `v-on` modifiers](/guide/migration/keycode-modifiers.html)
-- [$on, $off and \$once instance methods](/guide/migration/events-api.html)
+- [$on, $off and $once instance methods](/guide/migration/events-api.html)
 - [Filters](/guide/migration/filters.html)
 - [Inline templates attributes](/guide/migration/inline-template-attribute.html)
+- [`$children` intance property](/guide/migration/children.md)
 - `$destroy` instance method. Users should no longer manually manage the lifecycle of individual Vue components.
 
 ## Supporting Libraries