docs+nix: add deployment README and align release packaging

config/prod.exs

@@ -1,9 +1,3 @@
 import Config
 
-database_url =
-  System.get_env("DATABASE_URL") ||
-    raise "environment variable DATABASE_URL is missing. Example: ecto://USER:PASS@HOST/DATABASE"
-
-config :parrhesia, Parrhesia.Repo,
-  url: database_url,
-  pool_size: String.to_integer(System.get_env("POOL_SIZE") || "10")
+# Production runtime configuration lives in config/runtime.exs.