Adding button documentation
Adds buttons documentation to design.GitLab.com
Closes https://gitlab.com/gitlab-org/design.gitlab.com/issues/32 https://gitlab.com/gitlab-org/gitlab-ce/issues/29641
Edited by Dimitrie Hoekstra
Adds buttons documentation to design.GitLab.com
Closes https://gitlab.com/gitlab-org/design.gitlab.com/issues/32 https://gitlab.com/gitlab-org/gitlab-ce/issues/29641