[ecash] Add custom 404 page

Summary:
Currently we just have the generic next.js 404 page
Adding a custom one with more pizzazz and next steps for the user

Test Plan:
preview the site and go to the page that doesnt exist
should see the custom 404 page

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16358