[ImgBot] Optimize images (#961)

*Total -- 63.98kb -> 39.62kb (38.07%)

/docs/ds/images/vtree10.png -- 3.09kb -> 1.72kb (44.35%)
/docs/ds/images/vtree11.png -- 3.08kb -> 1.72kb (44.22%)
/docs/ds/images/vtree12.png -- 3.93kb -> 2.29kb (41.83%)
/docs/ds/images/vtree2.png -- 5.93kb -> 3.53kb (40.4%)
/docs/ds/images/vtree7.png -- 5.01kb -> 3.00kb (40.14%)
/docs/ds/images/vtree8.png -- 5.08kb -> 3.10kb (38.91%)
/docs/ds/images/vtree3.png -- 6.18kb -> 3.79kb (38.69%)
/docs/ds/images/vtree13.png -- 4.29kb -> 2.65kb (38.4%)
/docs/ds/images/vtree9.png -- 5.04kb -> 3.21kb (36.32%)
/docs/ds/images/vtree1.png -- 3.98kb -> 2.58kb (35.13%)
/docs/ds/images/vtree6.png -- 4.90kb -> 3.19kb (34.88%)
/docs/ds/images/vtree4.png -- 6.38kb -> 4.17kb (34.56%)
/docs/ds/images/vtree5.png -- 7.08kb -> 4.67kb (34.07%)