[website] Add FAQ to ecash page

Summary: Exchanges requested this info be provided to help users.

Test Plan: Read through the questions and answers. Ran the site locally and looked at it.

Reviewers: #bitcoin_abc, josephroyking

Reviewed By: #bitcoin_abc, josephroyking

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D9709