[test][is_valid] add as test case the polygon used in the documentation
example for is_valid
[doc][examples] change the color used for drawing the geometry
[doc][examples][is_simple] add image
[doc][examples][is_valid] add image; slightly modify the example geometry
(one of the interior rings was having CW orientation instead of CCW)
[doc][images] add PNG images for is_valid and is_simple documentation