Doc: Add generic thumbnails for the examples

As the examples do not have any images in their documentation,
no thumbnails are generated.

This change adds generic 'Qt Code Sample' thumbnails for the
examples, ensuring that they are listed in Qt Creator's
example list.

Change-Id: I40d3c69e9cebad72997d82eba0758c63c146aea7
Reviewed-by: Kurt Pattyn <pattyn.kurt@gmail.com>