[ImgBot] Optimize images (#1412)

*Total -- 412.00kb -> 334.04kb (18.92%)

/images/settings.png -- 12.04kb -> 5.53kb (54.07%)
/images/stirling-home-light.png -- 118.54kb -> 81.18kb (31.51%)
/images/login-light.png -- 30.08kb -> 24.98kb (16.94%)
/images/login-dark.png -- 29.44kb -> 25.32kb (13.98%)
/images/stirling-home.jpg -- 166.42kb -> 144.85kb (12.96%)
/docs/stirling-pdf.png -- 53.00kb -> 49.69kb (6.24%)
/src/main/resources/static/pdfjs/images/loading-icon.gif -- 2.49kb -> 2.48kb (0.35%)

Signed-off-by: ImgBotApp <ImgBotHelp@gmail.com>
Co-authored-by: ImgBotApp <ImgBotHelp@gmail.com>