[ImgBot] Optimize images

*Total -- 1,473.40kb -> 1,184.44kb (19.61%)

/images/pdf.png -- 210.49kb -> 169.21kb (19.61%)
/docs/extra-page/images/pdf.png -- 210.49kb -> 169.21kb (19.61%)
/docs/big-data/images/pdf.png -- 210.49kb -> 169.21kb (19.61%)
/docs/high-concurrency/images/pdf.png -- 210.49kb -> 169.21kb (19.61%)
/docs/distributed-system/images/pdf.png -- 210.49kb -> 169.21kb (19.61%)
/docs/micro-services/images/pdf.png -- 210.49kb -> 169.21kb (19.61%)
/docs/high-availability/images/pdf.png -- 210.49kb -> 169.21kb (19.61%)

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Author: ImgBotApp