TheCubicWebBook #111853 add howto for simple launch of existing application [open]

In the book, we start off straight away on the definition of a new cube. Have a simpler HOWTO that simply installs an existing useful application. For example create a forge. Simple, clear, works out of the box!

prioritynormal
typeenhancement
done in<not specified>
load0.500
load left0.500
closed by<not specified>