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