[website] Remove link to splitbch.com coin splitting service.

Summary:
The site is no longer there.
Also, add some more words around the Coinex method.

Test Plan:
checked that splitbch.com isn't available anymore
Built and tested site locally to look at page and test the links.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D9446