Implement the NATO converter with tests and Doxygen comments

Task: MIL-002#2
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src/nato_alphabet/domain/__init__.py

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+"""@brief Business rules of the NATO alphabet tool, free of input and output."""

src/nato_alphabet/domain/spelling.py

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+"""@brief Rule that spells a word with the NATO phonetic alphabet."""
+
+from collections.abc import Mapping
+
+
+class InvalidWordError(ValueError):
+    """@brief Raised when a word cannot be spelled.
+
+    A word is invalid when it is empty or holds a character that is not a
+    letter A to Z.
+    """
+
+
+def generate_phonetic(word: str, alphabet: Mapping[str, str]) -> list[str]:
+    """@brief Spell a word with the code word of each of its letters.
+
+    The match is case-insensitive.
+
+    @param word The word to spell.
+    @param alphabet Code word per upper-case letter.
+    @return The code words, one per letter of the word, in order.
+    @throws InvalidWordError The word is empty or holds a character that is
+        not a letter of the alphabet.
+    """
+    if not word:
+        raise InvalidWordError("The word is empty.")
+    try:
+        return [alphabet[letter.upper()] for letter in word]
+    except KeyError as error:
+        raise InvalidWordError(f"Not a letter: {error.args[0]!r}") from error