[website] Update coin splitting info on BCHA page.
Now that we have a blog post about it, refer to that.
Also clean up the wording around the other methods.
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