[ImgBot] Optimize images (#16)

*Total -- 1,327.74kb -> 745.11kb (43.88%)

/assets/opa-logo.png -- 275.81kb -> 59.41kb (78.46%)
/assets/test-file-structure-example.png -- 148.66kb -> 90.62kb (39.04%)
/assets/readme-test-results.png -- 172.19kb -> 107.78kb (37.41%)
/assets/readme-example-3.png -- 283.68kb -> 184.87kb (34.83%)
/assets/readme-example-2.png -- 164.27kb -> 108.10kb (34.19%)
/assets/readme-example-1.png -- 110.71kb -> 74.51kb (32.69%)
/assets/banner-pr-comment-example.png -- 172.43kb -> 119.83kb (30.5%)

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

Author: imgbot[bot]