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/time.nix

@@ -0,0 +1,15 @@
+{ config, pkgs, lib, ... }:
+
+{
+  # NTP servers for this environment
+  ntpServers = [
+    "10.10.128.1"    # IPv4 gateway
+    "fd00::1"        # IPv6 gateway
+  ];
+
+  # Time zone
+  timeZone = "Europe/Paris";
+
+  # Hardware clock setting
+  hardwareClock = "UTC";
+}