[website] Add more info to eCash page

Summary:
This is just a start based on the old BCHA page.
Tried to include known good links that are supporting the rebrand already.
Left out the exchanges for now, as their eCash support is very much in flux right now.

Test Plan: Ran locally, checked links

Reviewers: #bitcoin_abc, Fabien, PiRK

Reviewed By: #bitcoin_abc, Fabien, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D9723