refactor: move architecture and roadmap docs to local plan directory

These planning docs are not standard for Darts codebase. Moving to local
unversioned plan/ directory to keep for reference while maintaining only
user-facing documentation in docs/userguide/.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: nehalecky

.gitignore

@@ -26,3 +26,4 @@ docs_env
 
 # used by CI to build with latest versions of dependencies
 requirements-latest.txt
+/plan/