fix: correct import ordering in PlaceholderPageWriter

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: theletterf

src/services/Elastic.Documentation.Assembler/Building/PlaceholderPageWriter.cs

@@ -12,8 +12,8 @@
 using Elastic.Documentation.Navigation;
 using Elastic.Documentation.Navigation.V2;
 using Elastic.Documentation.Site;
-using Elastic.Documentation.Site.Navigation;
 using Elastic.Documentation.Site.FileProviders;
+using Elastic.Documentation.Site.Navigation;
 using Elastic.Markdown;
 using Elastic.Markdown.Page;
 using Microsoft.Extensions.Logging;