Purge sRGB chunks from PNGs in documentation.

Subjects each *.png file that matched grep -law "sRGB" to:
pngcrush -ow -brute -rem allb -reduce -force

Change-Id: I2ac34243ebe39c93860c5ac595c2f1eca4322eb8
Reviewed-by: Topi Reiniö <topi.reinio@theqtcompany.com>