[e.cash] Add build page

Summary:
Adding the start of the build page
To avoid files in /build getting ignored, named the app directory /build-on-ecash instead, and added next config rule to route
/build to that page

Test Plan: preview and check

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Subscribers: bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D18418