[ImgBot] Optimize images

*Total -- 424.65kb -> 231.39kb (45.51%)

/docs/imgs/ftlib_flow.png -- 198.00kb -> 61.36kb (69.01%)
/docs/imgs/ftlib.png -- 125.91kb -> 90.30kb (28.28%)
/docs/imgs/role_in_distributed.png -- 100.73kb -> 79.73kb (20.85%)

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