docs(mimic-iv): ensure documentation helper-query folder exists

Home for #1962 medication search SQL outside derived concepts.

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

Author: Chessing234

mimic-iv/concepts/documentation/README.md

@@ -0,0 +1,4 @@
+# Documentation helper queries
+
+Ad-hoc SQL that illustrates known data quirks. Not part of the official derived
+concept build.