Add an info panel regarding the chain split

Summary:
This helps educate users on why we have multi-chain releases.

Depends on a comms announcement.
Depends on D8290

Test Plan: make serve

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8291