Rename Developers page to Build on eCash
Summary: Part of T3493
Test Plan:
Run it locally, click around to see that everything is good.
Type in http://localhost:3000/developers to make sure it redirects properly
Reviewers: bytesofman, #bitcoin_abc, Fabien
Reviewed By: #bitcoin_abc, Fabien
Subscribers: Fabien
Differential Revision: https://reviews.bitcoinabc.org/D15895