element-properties - Rename file and id

Changes -

1. id - From "element-properties" to "element-child-content". Changed in file and sidebar.js

2. rename file - from "element-properties.mdx" to "element-child-content.mdx"

docs/element-child-content.mdx

@@ -1,6 +1,6 @@
 ---
-id: element-properties
-title: Element attributes and properties
+id: element-child-content
+title: Element properties - Child and Content
 ---
 
 import Admonition from "../components/Admonition"

sidebars.js

@@ -2,7 +2,7 @@ module.exports = {
   "Quick Guide": {
     "DOM and Events": [
       "create-modify-element",
-      'element-properties',
+      'element-child-content',
       'element-position-scroll',
      ],