[ImgBot] Optimize images

*Total -- 66.07kb -> 55.18kb (16.47%)

/.github/readme/images/slack-dark-128.png -- 6.45kb -> 4.18kb (35.3%)
/.github/readme/images/Layer5-MeshMentors.png -- 48.19kb -> 39.90kb (17.21%)
/assets/about-layer5.svg -- 1.63kb -> 1.48kb (9.16%)
/assets/social-icons/linkedIn.svg -- 0.61kb -> 0.58kb (4.82%)
/assets/social-icons/youtube.svg -- 0.69kb -> 0.66kb (4.27%)
/assets/social-icons/twitter.svg -- 0.86kb -> 0.83kb (3.41%)
/assets/social-icons/github.svg -- 0.95kb -> 0.92kb (3.08%)
/assets/social-icons/slack.svg -- 1.15kb -> 1.12kb (2.55%)
/assets/social-icons/email.svg -- 0.41kb -> 0.40kb (1.2%)
/assets/social-icons/discourse.svg -- 0.52kb -> 0.51kb (1.14%)
/assets/social-icons/docker.svg -- 1.92kb -> 1.92kb (0.25%)
/assets/meshmap-color-icon.svg -- 2.69kb -> 2.69kb (0.07%)

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

Author: ImgBotApp