feat: adds css-based skeleton loader

mentioned in commit f761281f

mentioned in merge request gitlab-org/frontend/pajamas-adoption-scanner!207 (merged)

mentioned in merge request status-page!874 (merged)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!3542 (merged)

mentioned in merge request gitlab-docs!4256 (merged)

mentioned in merge request gitlab-docs!4258 (closed)

mentioned in merge request gitlab!132987 (merged)