docs: add McGill Billboard Project hyperlink

Added the official DDMAL link at the first prose mention in README.md,
docs/architecture.md, and docs/glossary.md.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docs/architecture.md

@@ -411,7 +411,7 @@ key, prefix=None, temperature=1.0, top_p=0.9, max_tokens=300, seed=None)
 
 ### 3.9 `src/external_converters/mcgill_to_chord.py`
 
-**Назначение.** Конвертация аннотаций McGill Billboard Project в формат
+**Назначение.** Конвертация аннотаций [McGill Billboard Project](<https://ddmal.ca/research/The_McGill_Billboard_Project_(Chord_Analysis_Dataset)/>) в формат
 `.chord`.
 
 **Публичный интерфейс.**