[e.cash] Add FAQ to staking page

Summary:
Start with a couple questions that people have been asking.
Can add more as new questions arise

Test Plan:
Ran it locally
Read though the questions and answers to make sure they make sense

Reviewers: Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D14819