docs/manual: Modify GitHub FOO_SITE explanation

GitHub has changed its layout, there is no longer any green nor grey
button, it's a link with a tiny icon that indicates if it's uploaded by
the maintainer or not.
In order to not confuse anyone, adding an image should make it clear.

Signed-off-by: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Reviewed-by: Lionel Orry <lionel.orry@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>