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

src/libexpr/eval-settings.cc

@@ -92,7 +92,7 @@ bool EvalSettings::isPseudoUrl(std::string_view s)
 std::string EvalSettings::resolvePseudoUrl(std::string_view url)
 {
     if (hasPrefix(url, "channel:"))
-        return "https://nixos.org/channels/" + std::string(url.substr(8)) + "/nixexprs.tar.xz";
+        return "https://channels.nixos.org/" + std::string(url.substr(8)) + "/nixexprs.tar.xz";
     else
         return std::string(url);
 }

src/libexpr/primops.cc

@@ -2234,7 +2234,7 @@ static RegisterPrimOp primop_findFile(
       > - ```
       >   {
       >     prefix = "nixpkgs";
-      >     path = "https://nixos.org/channels/nixos-unstable/nixexprs.tar.xz";
+      >     path = "https://channels.nixos.org/nixos-unstable/nixexprs.tar.xz";
       >   }
       >   ```