Clinical zero-value audit for missing placeholders

Count zero values in Glucose, BloodPressure, SkinThickness, Insulin, and BMI.
Report zero percentages in a table and propose a clear handling rule for each of these features.

Start by locating the repository's Jupyter notebook cells that load the clinical dataset and define Glucose, BloodPressure, SkinThickness, Insulin, and BMI. Calculate each feature's zero percentage, present the results in a table, and document one clear handling rule for every feature; completion means all five features are covered.

Written by the indexing model from the issue text.