[website] Update coin splitting info on BCHA page.

Summary:
Now that we have a blog post about it, refer to that.
Also clean up the wording around the other methods.

Test Plan:
Built and viewed it locally
Read the info and make sure it's good.

Reviewers: jasonbcox, PiRK, #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D8532