docs(mimic-iv): sample query for chartevents before intime

Complement the after-outtime audit for #1961 sensitivity checks.

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

Author: Chessing234

mimic-iv/concepts/documentation/chartevents_before_intime.sql

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+-- Count chartevents charted before ICU intime (less common than after outtime).
+
+SELECT
+  COUNT(*) AS before_intime,
+  COUNT(*) FILTER (
+    WHERE ce.charttime < ie.intime
+      AND ce.charttime >= ie.intime - INTERVAL '2 hours'
+  ) AS before_intime_within_2h
+FROM icu.chartevents AS ce
+INNER JOIN icu.icustays AS ie
+  ON ce.stay_id = ie.stay_id
+WHERE ce.charttime < ie.intime;