[website] Add OKEX to list of exchanges

Summary: I have received confirmation that BCHA deposits and withdrawals are open at OKEX.

Test Plan: Ran locally, checked image and link.

Reviewers: #bitcoin_abc, majcosta, Fabien

Reviewed By: #bitcoin_abc, majcosta, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D9354