Update documentation and code snippets

docs/code_snippets/flags/greet_handler.py

@@ -0,0 +1,16 @@
+from argenta import Router, Response
+from argenta.command import Command, Flag
+
+router = Router()
+
+
+@router.command(Command("greet", flags=Flag("name")))
+def greet_handler(response: Response):
+    # Get flag by name
+    name_flag = response.input_flags.get_flag_by_name("name")
+
+    # Check if flag was passed
+    if name_flag:
+        print(f"Hello, {name_flag.input_value}!")
+    else:
+        print("Hello, stranger!")