restructure: rename termin to kapitel, flatten folder structure

- rename slide files: YYYY-MM-DD-termin-N-topic.md → NN-topic.md
- flatten folders: courses/X/slides/ → slides/X/
- replace "Termin" with "Kapitel" in all content
- add klausur extraction script (make klausur)
- update Makefile, generate-index.sh, dev-server.sh
- add README.md with full documentation

scripts/dev-server.sh

@@ -8,7 +8,7 @@ set -e
 INDEX_PORT=1310
 COURSE_B_PORT=1311
 COURSE_C_PORT=1312
-COURSE_DIR="courses"
+SLIDES_DIR="slides"
 DEV_INDEX_DIR=".dev-index"
 
 # Colors
@@ -94,12 +94,12 @@ echo ""
 python3 -m http.server $INDEX_PORT --directory "$DEV_INDEX_DIR" &
 PID_INDEX=$!
 
-PORT=$COURSE_B_PORT npx @marp-team/marp-cli --server "$COURSE_DIR/223015b/slides/" &
+PORT=$COURSE_B_PORT npx @marp-team/marp-cli --server "$SLIDES_DIR/223015b/" &
 PID_B=$!
 
 sleep 3  # Stagger starts to avoid WebSocket port collision
 
-PORT=$COURSE_C_PORT npx @marp-team/marp-cli --server "$COURSE_DIR/223015c/slides/" &
+PORT=$COURSE_C_PORT npx @marp-team/marp-cli --server "$SLIDES_DIR/223015c/" &
 PID_C=$!
 
 # Wait for any process to exit