[ImgBot] Optimize images

*Total -- 550.40kb -> 260.16kb (52.73%)

/docs/a.png -- 128.74kb -> 40.94kb (68.19%)
/docs/b.png -- 133.51kb -> 68.58kb (48.63%)
/docs/screenshot.png -- 288.16kb -> 150.63kb (47.73%)

Signed-off-by: ImgBotApp <ImgBotHelp@gmail.com>

Author: ImgBotApp