[e.cash] Add Pay2Stay to Use eCash page

Summary: Pay2Stay are launching and promoting themselves, so good time to add them to the site.

Test Plan: Ran it locally, checked the link.

Reviewers: #bitcoin_abc, Kronkmeister

Reviewed By: #bitcoin_abc, Kronkmeister

Subscribers: johnkuney, Kronkmeister, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17344