Add Caesar cipher project with Doxygen docs

src/caesar_cipher/constants.py

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+## @file constants.py
+#  @brief Constants used by the Caesar cipher.
+
+## Lowercase alphabet used to look up letter positions.
+ALPHABET = list("abcdefghijklmnopqrstuvwxyz")
+
+## Prompt for the message.
+PROMPT_TEXT = "Type your message:\n"
+## Prompt for the shift number.
+PROMPT_SHIFT = "Type the shift number:\n"
+
+## Name of the environment file holding tokens.
+ENV_FILE = ".env"