[ImgBot] Optimize images

*Total -- 1,320.28kb -> 935.23kb (29.16%)

/readme_assets/mock.png -- 953.92kb -> 649.97kb (31.86%)
/readme_assets/readme_banner.png -- 180.53kb -> 137.21kb (24%)
/readme_assets/readme_banner-light.png -- 185.83kb -> 148.06kb (20.33%)

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

Author: ImgBotApp