Prepare a 502 page for PaaS Docker

Summary:
This external page is intended to be commited into rDII.

The external pages are put in the src/page/_external folder
and discarded during the build before deployment.

See for example 726fa910fe44.

Test Plan:

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D1991