[website] update old BCHA links

Summary:
Coinex link noted in review for D10968
Also update another link from old BCHA page to eCash page

Test Plan: ran locally and clicked on the links to check them

Reviewers: PiRK, #bitcoin_abc

Reviewed By: PiRK, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D10981