Added Illustrations

content/design.md

layouts/design/single.html

layouts/partials/design/icons.html

layouts/partials/design/illustrations.html

static/images/chat_doodle_black.png

static/images/chat_doodle_black.svg

static/images/chat_doodle_subtle.png

static/images/chat_doodle_subtle.svg

static/images/chat_doodle_subtle_dark.png

static/images/chat_doodle_subtle_dark.svg

static/images/chat_doodle_subtle_lime_dark.png

static/images/chat_doodle_subtle_lime_dark.svg