[website] Add new coin splitting article

Summary: Updated version of https://blog.bitcoinabc.org/2020/11/25/how-to-split-your-bch-bchn-and-bcha-coins-using-electrum-abc-and-electron-cash/

Test Plan:
I successfully followed the steps in the article to split coins.
Built the site locally, read throught the article, and checked links.

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: PiRK

Maniphest Tasks: T2209

Differential Revision: https://reviews.bitcoinabc.org/D10968