[website] Move old rebrand info to BCHA page

Summary: Most users won't need this info, but some may. So move to it's own page.

Test Plan: Built it locally, read through both pages to make sure they make sense.

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D10237