[ImgBot] Optimize images

*Total -- 680.93kb -> 459.55kb (32.51%)

/imgs/branching_strategy.png -- 207.62kb -> 124.74kb (39.92%)
/imgs/branching_strategy-white_background.png -- 207.62kb -> 124.74kb (39.92%)
/imgs/branching_strategy-demo.png -- 150.17kb -> 98.40kb (34.47%)
/imgs/branching_strategy-demo-white_background.png -- 115.52kb -> 111.67kb (3.33%)

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

Author: ImgBotApp