feat(datepicker): add public show method for datepicker

mentioned in commit 5c256e55

mentioned in merge request status-page!625 (closed)

mentioned in merge request gitlab!101292 (merged)

mentioned in merge request status-page!626 (merged)

mentioned in merge request gitlab-docs!3223 (merged)

mentioned in merge request gitlab-docs!3225 (merged)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!3108 (closed)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!3109 (merged)

mentioned in merge request gitlab-org/frontend/pajamas-adoption-scanner!87 (merged)