Doc: Move and edit examples page

Changed the example page type to a group to have a standardized
table of examples. Also, a filename 'examples.html' is too
collision-prone when Qt docs are output to a single directory.

Also, moved the examples.qdoc file into /doc subdirectory to
avoid it from being copied to QT_INSTALL_EXAMPLES when packaging
the examples.

Change-Id: I87251e0fde127bdef3024907b6472b50039f0357
Reviewed-by: Kurt Pattyn <pattyn.kurt@gmail.com>
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>