Add the Marlin Wallet website

Summary: This is simple static html/css, nothing fancy.

Test Plan:
Open index.html in the browser.
It's deployed already at https://marlinwallet.com

Reviewers: #bitcoin_abc, Mengerian

Reviewed By: #bitcoin_abc, Mengerian

Subscribers: aimal

Differential Revision: https://reviews.bitcoinabc.org/D19007