add makefile improvements and index generator

- add deploy-html, optimize-images, copy-assets targets
- add scripts/generate-index.sh for slide overview
- integrate index generation into html/build targets
- deploy to 223015c path

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.gitignore

@@ -63,3 +63,4 @@ Desktop.ini
 *.pdf
 index.md.bak
 hdm-lehrauftrag-uebersicht.md
+slides/assets-original/