[e.cash] Add Subnets as a key feature

Summary:
Replaces "small convenient demonination" on front page

T3478

Test Plan:
Run it locally ot make sure it looks good.
Read through the copy to make sure it's good.

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Subscribers: bytesofman

Maniphest Tasks: T3478

Differential Revision: https://reviews.bitcoinabc.org/D15805