[e.cash] Fix video background not showing

Summary:
on subpages such as /core-tech or /roadmap the blue video background in the top section was hidden due to a css issue
Addings some z-index adjustments so it works on subpages and the landing page

Test Plan: preview the site and check the video background looks good

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16748