[ImgBot] Optimize images

*Total -- 2,665.59kb -> 1,225.54kb (54.02%)

/docs/a.png -- 128.74kb -> 40.94kb (68.19%)
/docs/streamlit-markdown.gif -- 2,115.19kb -> 965.38kb (54.36%)
/docs/b.png -- 133.51kb -> 68.58kb (48.63%)
/docs/screenshot.png -- 288.16kb -> 150.63kb (47.73%)

Signed-off-by: ImgBotApp <[email protected]>

Author: ImgBotApp