[e.cash] Add Agora library to build page

Summary: adding Agora npm library to the 'library' section on e.cash/build

Test Plan: visit e.cash/build, find ecash-agora under libraries, check the link, and check for typos.

Reviewers: #bitcoin_abc, Mengerian

Reviewed By: #bitcoin_abc, Mengerian

Differential Revision: https://reviews.bitcoinabc.org/D18499