docs: add check_pangram with doctests

Implement a function to check if a string is a pangram.

Author: farhan493495

strings/check_pangram.py

@@ -0,0 +1,16 @@
+def check_pangram(input_str: str) -> bool:
+    """
+    Check if a string is a pangram (contains every letter of the alphabet).
+    >>> check_pangram("The quick brown fox jumps over the lazy dog")
+    True
+    >>> check_pangram("Hello World")
+    False
+    >>> check_pangram("")
+    False
+    """
+    return len(set(c for c in input_str.lower() if c.isalpha())) == 26
+
+
+if __name__ == "__main__":
+    import doctest
+    doctest.testmod()