feat(CSS): define basic styles for <kbd> element

mentioned in commit 5eeded83

mentioned in merge request gitlab-svgs!1171 (merged)

mentioned in merge request status-page!903

mentioned in merge request gitlab!191470 (merged)

mentioned in merge request gitlab!191777 (merged)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!4433 (merged)

mentioned in merge request gitlab-org/technical-writing/docs-gitlab-com!740 (merged)

mentioned in merge request gitlab-org/technical-writing/docs-gitlab-com!741 (merged)

mentioned in merge request gitlab-org/editor-extensions/gitlab-jetbrains-plugin!1731 (merged)