split index.md into modular slide files

- extract _frontmatter.md, _intro.md, _outro.md as shared components
- create separate files per termin with date-topic naming convention:
  2025-12-19-termin-1-grundlagen-text-audio.md
  2026-01-09-termin-2-bild-audio-video.md
  2026-01-23-termin-3-speichermedien-schnittstellen.md
  2026-01-30-termin-4-distribution-apis-zukunft.md
  2026-xx-xx-termin-5-vertiefung-offene-fragen.md
- keep index.md.bak as backup (gitignored)

.gitignore

@@ -61,3 +61,4 @@ Desktop.ini
 *.html
 !build/*.html
 *.pdf
+index.md.bak