[e.cash] Update the mining page

Summary: Improve the instructions to get started with mining eCash and include Heartbeat.

Test Plan: Build the preview and check the rendering.

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Subscribers: Mengerian, bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D16834