Docs: losslessly compress images.

Before: 25,1 KB (25.773 bytes)
After:  23,7 KB (24.352 bytes)
Compress examples screenshots with kraken.io.

Before: 1,12 MB (1.174.728 bytes)
After:  1,11 MB (1.173.614 bytes)
Merge pull request #12465 from XhmikosR/docs-img

Docs: losslessly compress images.