[website] Migrate mining article to site blog

Summary:
In preparation for deprecating the Wordpress blog, copy useful articles over
Original article here: https://blog.bitcoinabc.org/2021/07/06/mining-ecash-using-viabtc-mining-pool/

Test Plan: ran it locally and compared to Wordpress version. Looks the same.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10996