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: I621cb2437578c3f8a658e7c26a4de045825147ce
Reviewed-by: Topi Reiniö <topi.reinio@theqtcompany.com>