[ImgBot] Optimize images

*Total -- 2,116.68kb -> 1,589.56kb (24.9%)

/images/article-demo.png -- 545.54kb -> 379.90kb (30.36%)
/docs/extra-page/images/article-demo.png -- 545.54kb -> 379.90kb (30.36%)
/images/qrcode_for_doocs.jpg -- 27.29kb -> 20.35kb (25.42%)
/docs/extra-page/images/qrcode_for_doocs.jpg -- 27.29kb -> 20.35kb (25.42%)
/images/wechat-group-for-doocs.png -- 485.52kb -> 394.53kb (18.74%)
/docs/extra-page/images/wechat-group-for-doocs.png -- 485.52kb -> 394.53kb (18.74%)
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