Simiplify sidebar structure for DOM Manipulation

I have called this whole section as "DOM manipulation" (instead of DOM and events), so all the sub-files can be named easily with that context, without prefixing with "element-" or "DOM".

Renamed "DOM manipulation" page title to "Creating elements", then shortended title for "Child and Content" file by removing prefix.

docs/create-modify-element.mdx

@@ -1,7 +1,6 @@
 ---
 id: create-modify-element
-sidebar_label: DOM Manipulation
-title: Basic DOM Manipulation
+title: Creating elements
 slug: /
 ---
 

docs/element-child-content.mdx

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