Add Managing your System cover as well as Changing Background doc and media.

content/en/managing-your-system.md

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+# Managing your system
+
+<!-- Reference managing-your-system/* contents. -->

content/en/managing-your-system/changing-background.md

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+## Changing Background
+
+![Background selector in Gnome Control Center](../../images/managing-your-system/changing-background.png)
+
+You can change your background by going to the Budgie Menu, opening Settings, then clicking Background.
+
+You will have two options, Background and Lock Screen. Select Background.
+
+In this section, you can choose between predefined wallpapers,pictures in your Pictures folder, or solid colors. After choosing, click Select.