Update schedule pipeline layout

What does this MR do and why?

• Move cron help text underneath input
• Add spacing between buttons, stack buttons on mobile
• Remove hr on Run pipeline (Pulled over from Draft: Update run pipeline page layout (!131670 - closed))

Screenshots or screen recordings

Before	After

How to set up and validate locally

http://localhost:3000/gitlab-org/gitlab-test/-/pipeline_schedules/new

UX Paper Cuts 16.4 → Pipelines and Jobs (#408231 - closed), UX Paper Cuts 16.5 → Cleanup (#424403 - closed)