[website] mention appimaged and AppImageUpdate

https://github.com/Ultimaker/Cura/issues/2634#issuecomment-351826233 raises a good point:

> why isn't appimaged (and AppImageUpdate) mentioned on appimage.org?

Proposed text:

**UP TO DATE**

Using [AppImageUpdate](https://github.com/AppImage/AppImageUpdate) functionality, you can update applications by downloading only these portions that have changed since the version you are currently running. This makes updating very fast and efficient, and particularly interesting for continuous or nightly builds.
- Users can use the optional AppImageUpdate standalone application.
- Developers can include AppImageUpdate functions into their applications directly so that users don't even need a separate AppImageUpdate standalone application.

**INTEGRATED INTO YOUR DESKTOP**

Using the optional `appimaged` daemon, you can have AppImages automatically integrated into your desktop's menus: it will add icons and file type associations to your desktop automatically:
- You download an AppImage and it shows up in your menu.
- You delete the AppImage, and it is gone from the menu.
- AppImage files show the icons of the respective applications.

Some Linux distributions (such as [Nitrux OS](https://nxos.org/)) even come preconfigured for this out-of-the-box. If yours does not, you can download it [here](https://github.com/AppImage/AppImageKit/releases). It does not need 'root' privileges to run and work for you.