[website] Add "what is the supply" to eCash FAQ

Summary:
Explain the 21 Trillion supply limit, and how it's equivalent to Bitcoin's limit

Ref: T1825

Test Plan:
Built and ran it locally.
read it over the make sure it makes sense.

Reviewers: #bitcoin_abc, josephroyking, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D10045