Add Caesar cipher project with Doxygen docs

src/caesar_cipher/main.py

@@ -0,0 +1,20 @@
+## @file main.py
+#  @brief Interactive command line interface.
+
+from caesar_cipher import constants as c
+from caesar_cipher.cipher import encrypt
+
+
+def main() -> None:
+    ## @brief Ask for a message and a shift, then print the encrypted text.
+    text = input(c.PROMPT_TEXT)
+    try:
+        shift = int(input(c.PROMPT_SHIFT))
+    except ValueError:
+        print("The shift must be a whole number.")
+        return
+    print(f"Here's the encoded result: {encrypt(text, shift)}")
+
+
+if __name__ == "__main__":
+    main()