[e.cash] Add link for Chronik docs to build page

Summary: Docs are now available, let's let the builders find them

Test Plan: Run it locally, check that it looks OK and link works

Reviewers: #bitcoin_abc, johnkuney

Reviewed By: #bitcoin_abc, johnkuney

Subscribers: johnkuney

Differential Revision: https://reviews.bitcoinabc.org/D16873