[website] Add be.cash explorer to ecash page

Summary: It has implemented the rebrand

Test Plan:
ran it locally, checked link.
Poked around the be.cash explorer to check that address and denomination are correct.

Reviewers: #bitcoin_abc, josephroyking

Reviewed By: #bitcoin_abc, josephroyking

Differential Revision: https://reviews.bitcoinabc.org/D9796