[marlin-website] Enforce bottom padding in the hero

Summary:
This prevents the QR code from aligning with the background boundary which looks weird on smaller screens.

Depends on D19026.

Test Plan: Open index.html, reduce the screen size and check the padding applies.

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D19027