[ImgBot] Optimize images (#2480)

*Total -- 1,282.23kb -> 875.14kb (31.75%) /assets/add-remote.png -- 129.57kb -> 67.05kb (48.26%) /assets/gitui-signing.png -- 36.67kb -> 22.29kb (39.22%) /assets/logo.png -- 55.26kb -> 35.84kb (35.14%) /assets/syntax-highlighting-blame.png -- 1,060.72kb -> 749.97kb (29.3%) Signed-off-by: ImgBotApp <ImgBotHelp@gmail.com> Co-authored-by: ImgBotApp <ImgBotHelp@gmail.com>

Author: imgbot[bot]