Add Wortschatz & Grammatik A1 Abschnitt A, switch Ergaenzung to per-section filing

Uses the book's own lettered Inhalt structure (Abschnitt A-Q) instead of
per-sub-exercise files, since sub-exercises are often half a page each.
Section A covers Person, Familie und Freunde (pp. 8-18), cross-referenced
against Menschen Lektion 1 and 3.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Ergaenzung/_template.md

@@ -1,6 +1,10 @@
-# [Book] — Einheit XX: [Title/Topic]
+# [Book] — Abschnitt X: [Title/Topic]
 
-Pages: [XX–YY]
+Pages: [XX–YY] · Sub-exercises: [e.g. A1–A17]
+
+Note: file one section per book-defined chapter/letter (matching its own
+Inhaltsverzeichnis), not per tiny sub-exercise — summarize sub-exercises
+inside this file's Exercise summary instead.
 
 ## Overview