Change channel URLs to channels.nixos.org subdomain

Update all channel URLs from https://nixos.org/channels/ to
https://channels.nixos.org/ to use the more reliable subdomain.

The nixos.org domain apex lacks IPv6 support due to DNS hoster
limitations. Using the subdomain allows better CDN distribution
and improved reliability.

Updated files:
- Installation scripts (multi-user and tarball installers)
- Channel URL resolution in eval-settings.cc
- Documentation and examples
- Docker image default channel URL
- Release notes (added note about URL change)

Fixes #14517

scripts/install-multi-user.sh

@@ -714,7 +714,7 @@ EOF
 
 place_channel_configuration() {
     if [ -z "${NIX_INSTALLER_NO_CHANNEL_ADD:-}" ]; then
-        echo "https://nixos.org/channels/nixpkgs-unstable nixpkgs" > "$SCRATCH/.nix-channels"
+        echo "https://channels.nixos.org/nixpkgs-unstable nixpkgs" > "$SCRATCH/.nix-channels"
         _sudo "to set up the default system channel (part 1)" \
             install -m 0644 "$SCRATCH/.nix-channels" "$ROOT_HOME/.nix-channels"
     fi