[e.cash] Add redirect for /roadmap-explained

Summary:
adding redirect for /roadmap-explained
as outlined here https://nextjs.org/docs/pages/api-reference/next-config-js/redirects

Test Plan: preview the site and check /roadmap-explained redirects to /roadmap

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien, bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D14305