docs(mimic-iv-ed): add concepts folder for helper queries

Keep #1910 audit SQL out of the official derived-concept tree.

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: Chessing234

mimic-iv-ed/concepts/README.md

@@ -0,0 +1,5 @@
+# MIMIC-IV-ED helper concepts
+
+Small documentation-oriented queries that illustrate known data quirks.
+These are **not** part of the official MIMIC-IV derived concept build
+(`mimic-iv/concepts`); run them ad hoc against a local or BigQuery copy.