[e.cash] Add max height to image styles to fix image on what's eCash page

Summary:
Adding an explicit height of 830px; to HeroCtn in sub-page-hero styles and removing the min height.
The spiningcoin animation was ranging into the navbar, looks cleaner now.

Test Plan:
visit e.cash/what-is-eCash, check positioning of spiningcoin animation.
Also check other pages for images being rendered correctly.

Reviewers: #bitcoin_abc, bytesofman, johnkuney

Reviewed By: #bitcoin_abc, bytesofman, johnkuney

Subscribers: johnkuney, bytesofman, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16634