feat: update sample environment to match nixos-infra structure

- Add network/dns.nix, network/time.nix, network/proxy.nix, network/smtp.nix
- Add network/default.nix to import all network files
- Add hosts/default.nix, hosts/servers/default.nix, hosts/workstations/default.nix
- These placeholders match the structure in nixos-infra for consistency

The sample environment now reflects the same organization as production,
dev, and stage environments, making it easier to use as a reference.

environments/sample/network/smtp.nix

@@ -0,0 +1,12 @@
+{ config, pkgs, lib, ... }:
+
+{
+  # SMTP relay configuration for this environment
+  relayHost = "smtp.lagraula.fr";
+  relayPort = 587;
+
+  useTLS = true;
+  useSTARTTLS = true;
+
+  fromAddress = "noreply@sample.lagraula.fr";
+}