Ignore the dotnet publish output directory

BUILDING.md and ci.yml both publish self-contained builds into out/, but
nothing ignored it. bin/ and obj/ already are; out/ was the one build output
location left uncovered, and it holds a full .sdPlugin folder - hundreds of
runtime DLLs and native libraries that have no business in source control.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TT9jGty9jKfDTrpGnwV3DR

Author: TheJoeIaut

.gitignore

@@ -32,6 +32,10 @@ bld/
 [Ll]og/
 [Ll]ogs/
 
+# dotnet publish output (see BUILDING.md / ci.yml) - a built .sdPlugin folder,
+# never something to commit.
+/out/
+
 # Visual Studio 2015/2017 cache/options directory
 .vs/
 # Uncomment if you have tasks that create the project's static files in wwwroot