docs: Optimize images

*Total -- 565.38kb -> 223.62kb (60.45%)

/screenshots/month.png -- 77.30kb -> 25.36kb (67.2%)
/screenshots/hour.png -- 180.05kb -> 61.47kb (65.86%)
/screenshots/year.png -- 63.71kb -> 22.17kb (65.21%)
/screenshots/minute.png -- 99.48kb -> 34.69kb (65.13%)
/screenshots/day.png -- 143.95kb -> 79.16kb (45.01%)
/images/coverage-badge.svg -- 0.87kb -> 0.78kb (10.18%)

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

Author: imgbot[bot]