[ImgBot] Optimize images

*Total -- 528.51kb -> 445.29kb (15.75%) /docs/openwhisk-nav.png -- 31.09kb -> 24.33kb (21.75%) /docs/OpenWhisk-MessageHub-sample-architecture.png -- 128.21kb -> 105.89kb (17.41%) /docs/getting-started-with-openwhisk.png -- 74.83kb -> 61.92kb (17.26%) /docs/openwhisk-cli.png -- 294.38kb -> 253.16kb (14%)

Author: ImgBotApp