Update design.GitLab.com with new brand assets and content

Changes

Landing page:

• Logo and text updated
• Cover gradient updated
• Principles removed (they'll have a dedicated page with https://gitlab.com/gitlab-org/design.gitlab.com/merge_requests/185)
• Text and illustration added explaning intent and usage of pajamas

Sidebar:

• Logo updated with official GitLab logo + hover animation

Screenshot