Update docs/pandas_cheatsheet.md

docs/pandas_cheatsheet.md

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 # Pandas Cheatsheet
 
-Pandas is a Python library used for data analysis.
-
-## 🧱 Data Structures
-- `Series`: 1D array
-- `DataFrame`: 2D table (like Excel)
-
-## 📥 Read Data
-```python
-import pandas as pd
-df = pd.read_csv("data.csv")
+- `df.head()` – Show first rows  
+- `df.describe()` – Summary stats  
+- `df.isnull().sum()` – Count missing values  
+- `df.fillna(value)` – Fill missing