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/dns.nix

@@ -0,0 +1,21 @@
+{ config, pkgs, lib, ... }:
+
+{
+  # Sample DNS configuration for the sample environment
+  domain = "sample.lagraula.fr";
+
+  # Default DNS servers for this environment
+  defaultNameServers = [ "10.40.128.10" ];
+
+  # Forwarders for this environment
+  forwarders = [ "1.1.1.1" "8.8.8.8" ];
+
+  # Allow zone transfers (none in sample)
+  allowZoneTransfer = [ ];
+
+  # Recursion policy
+  recursion = "AllowOnlyForPrivateNetworks";
+
+  # Email for Let's Encrypt
+  letsEncryptEmail = "xavier@lagraula.fr";
+}