[e.cash] Add whitepaper link to hero

Summary: Adding the bitcoin whitepaper to the site and linking to it from the nakamoto box in the hero section of homepage as requested

Test Plan: preview and check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18730