Adding data viz documentation
First pass at adding information from gitlab-org/gitlab-ui#257 (closed) into design.gitlab.com. This MR would also address: https://gitlab.com/gitlab-org/design.gitlab.com/issues/316
First pass at adding information from gitlab-org/gitlab-ui#257 (closed) into design.gitlab.com. This MR would also address: https://gitlab.com/gitlab-org/design.gitlab.com/issues/316