OTP application for the deployable Shadix docs site (compiled only outside
:prod; wired as the release mod: only when MIX_ENV=docs).
Mirrors the supervision tree booted ad-hoc by dev.exs for local work, but
here it is started by the release so the endpoint comes up under the OTP
supervisor. Endpoint configuration comes from config/docs.exs (compile time)
and config/runtime.exs (PORT / SECRET_KEY_BASE / PHX_HOST at boot).