MIL-002 NATO converter: modelling, converter code, tests and Doxygen comments

User story, use case and SSD for "spell a word"; spell_word with
try/except KeyError, generate_phonetic with the retry loop, console and CSV
adapters, 49 pytest tests; MIL-002 and the modelling documents accepted with
review records.

Task: MIL-002#1
Task: MIL-002#2
Task: MIL-002#3
Task: MIL-002#4
Task: MIL-002#5
Task: MIL-002#6

src/nato_phonetic_alphabet/__main__.py

@@ -0,0 +1,5 @@
+"""@brief Entry point of the command `python -m nato_phonetic_alphabet`."""
+
+from nato_phonetic_alphabet.main import main
+
+raise SystemExit(main())