[website] Migrate eCash rebrand announcement to site blog

Summary:
This is in preparation for deprecating the Wordpress blog.
Original article at: https://blog.bitcoinabc.org/2021/06/28/ecash-wealth-redefined/

Test Plan: Ran it locally, looks ok

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10995