Adding a landing page for BCHA information

Summary:
We need a landing page to hold the various information users might need about BCHA.
This is a very quick and basic version of that, but will continue to improve and expand as needed.

Test Plan: Review and iterate as needed

Reviewers: #bitcoin_abc, deadalnix, Mengerian

Reviewed By: #bitcoin_abc, deadalnix, Mengerian

Subscribers: Mengerian, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D8476