Add find unused markdown files workflow

[Nirus2000]

  [de] Empty file 'how-to/gold-preise.md' (Markdown file is empty or contains only front matter)
  [de] Empty file 'how-to/xml bearbeiten.md' (Markdown file is empty or contains only front matter)
  [de] Empty file 'how-to/wahldividenden.md' (Markdown file is empty or contains only front matter)
  [de] Empty file 'how-to/morningstar.md' (Markdown file is empty or contains only front matter)
  [de] Empty file 'how-to/bewertung.md' (Markdown file is empty or contains only front matter)
  [de] Empty file 'how-to/zusammenschluss-zweier-firmen.md' (Markdown file is empty or contains only front matter)
  [de] Empty file 'how-to/aktien-splitt.md' (Markdown file is empty or contains only front matter)
  [de] Empty file 'how-to/insolvenzen.md' (Markdown file is empty or contains only front matter)
  [de] Empty file 'how-to/reconstruct-sample-portfolio.md' (Markdown file is empty or contains only front matter)
  [de] Empty file 'how-to/wertpapiere-kopieren.md' (Markdown file is empty or contains only front matter)
  [de] Empty file 'how-to/spin-off.md' (Markdown file is empty or contains only front matter)
  [de] Empty file 'how-to/benutzeroberflaeche.md' (Markdown file is empty or contains only front matter)
  [de] Empty file 'how-to/import-gbx.md' (Markdown file is empty or contains only front matter)
  [de] Empty file 'grundlagen/einstandspreis.md' (Markdown file is empty or contains only front matter)
  [de] Empty file 'grundlagen/berichtszeitraum.md' (Markdown file is empty or contains only front matter)
  [de] Empty file 'grundlagen/performance/zeit-gewichtet.md' (Markdown file is empty or contains only front matter)
  [de] Empty file 'grundlagen/performance/geld-gewichtet.md' (Markdown file is empty or contains only front matter)
  [de] Empty file 'referenzhandbuch/index.md' (Markdown file is empty or contains only front matter)
  [de] Empty file 'referenzhandbuch/online.md' (Markdown file is empty or contains only front matter)
  [de] Empty file 'referenzhandbuch/ansichten/optionen.md' (Markdown file is empty or contains only front matter)
  [de] Empty file 'referenzhandbuch/ansichten/berichte/performance/index.md' (Markdown file is empty or contains only front matter)
  [de] Empty file 'referenzhandbuch/ansichten/berichte/performance/performance-diagramm.md' (Markdown file is empty or contains only front matter)
  [de] Empty file 'referenzhandbuch/ansichten/berichte/performance/berechnung.md' (Markdown file is empty or contains only front matter)
  [de] Empty file 'referenzhandbuch/ansichten/berichte/performance/trades.md' (Markdown file is empty or contains only front matter)
  [de] Empty file 'referenzhandbuch/ansichten/berichte/performance/zahlungen.md' (Markdown file is empty or contains only front matter)
  [de] Empty file 'referenzhandbuch/ansichten/berichte/performance/wertpapiere.md' (Markdown file is empty or contains only front matter)
  [de] Empty file 'referenzhandbuch/ansichten/berichte/vermoegensaufstellung/index.md' (Markdown file is empty or contains only front matter)
  [de] Empty file 'referenzhandbuch/ansichten/berichte/vermoegensaufstellung/bestand.md' (Markdown file is empty or contains only front matter)
  [de] Empty file 'referenzhandbuch/ansichten/berichte/vermoegensaufstellung/vermoegensaufstellung-diagramm.md' (Markdown file is empty or contains only front matter)
  [de] Empty file 'referenzhandbuch/ansichten/stammdaten/index.md' (Markdown file is empty or contains only front matter)
  [de] Empty file 'referenzhandbuch/ansichten/stammdaten/konten.md' (Markdown file is empty or contains only front matter)
  [de] Empty file 'referenzhandbuch/ansichten/stammdaten/depots.md' (Markdown file is empty or contains only front matter)
  [de] Empty file 'referenzhandbuch/ansichten/stammdaten/sparplaene.md' (Markdown file is empty or contains only front matter)
  [de] Empty file 'referenzhandbuch/ansichten/stammdaten/alle-transaktionen.md' (Markdown file is empty or contains only front matter)
  [de] Empty file 'referenzhandbuch/ansichten/stammdaten/waehrungen.md' (Markdown file is empty or contains only front matter)
  [de] Empty file 'referenzhandbuch/ansichten/wertpapiere/alle-wertpapiere.md' (Markdown file is empty or contains only front matter)
  [de] Empty file 'referenzhandbuch/ansichten/wertpapiere/kontext-menue.md' (Markdown file is empty or contains only front matter)
  [de] Empty file 'referenzhandbuch/transaktionen/index.md' (Markdown file is empty or contains only front matter)
  [de] Empty file 'referenzhandbuch/transaktionen/kauf-verkauf.md' (Markdown file is empty or contains only front matter)
  [de] Empty file 'referenzhandbuch/transaktionen/zinsen-zinsbelastung.md' (Markdown file is empty or contains only front matter)
  [de] Empty file 'referenzhandbuch/transaktionen/wertpapier-umbuchen.md' (Markdown file is empty or contains only front matter)
  [de] Empty file 'referenzhandbuch/transaktionen/steuern-gebuehren.md' (Markdown file is empty or contains only front matter)
  [de] Empty file 'referenzhandbuch/transaktionen/dividende.md' (Markdown file is empty or contains only front matter)
  [de] Empty file 'referenzhandbuch/transaktionen/einlage-entnahme.md' (Markdown file is empty or contains only front matter)
  [de] Empty file 'referenzhandbuch/transaktionen/ein-auslieferung.md' (Markdown file is empty or contains only front matter)
  [de] Empty file 'referenzhandbuch/hilfe/fehlerprotokoll.md' (Markdown file is empty or contains only front matter)
  [de] Empty file 'referenzhandbuch/hilfe/ueber.md' (Markdown file is empty or contains only front matter)
  [de] Empty file 'referenzhandbuch/allgemeine-daten/einstellungen.md' (Markdown file is empty or contains only front matter)
  [de] Empty file 'referenzhandbuch/allgemeine-daten/waehrungen.md' (Markdown file is empty or contains only front matter)
  [de] Empty file 'referenzhandbuch/datei/import.md' (Markdown file is empty or contains only front matter)
  [de] Empty file 'referenzhandbuch/datei/speichern.md' (Markdown file is empty or contains only front matter)
  [de] Empty file 'referenzhandbuch/datei/export.md' (Markdown file is empty or contains only front matter)
  [de] Empty file 'referenzhandbuch/datei/waehrungen.md' (Markdown file is empty or contains only front matter)

[buchen]

I believe there is an ongoing discussion how to best handle the case where we want to motivate contributors to write content. And one way is to have a placeholder page and point to the "edit this page" button. We do not decide this here and we do not start failing the build (and stopping the workflow form any changes anymore).

[buchen]

I converted it to draft for the time being.