feat: Add PPTX notes slides (#474)

* feat: Add PPTX notes slides

Presenter notes may have useful information and should also be extracted.

Signed-off-by: Maciej Wieczorek <maciej@wieczorek.co>

* feat: Move presenter notes into furniture

Signed-off-by: Maciej Wieczorek <maciej@wieczorek.co>

---------

Signed-off-by: Maciej Wieczorek <maciej@wieczorek.co>

docling/backend/mspowerpoint_backend.py

@@ -16,6 +16,7 @@ from docling_core.types.doc import (
     TableCell,
     TableData,
 )
+from docling_core.types.doc.document import ContentLayer
 from PIL import Image, UnidentifiedImageError
 from pptx import Presentation
 from pptx.enum.shapes import MSO_SHAPE_TYPE, PP_PLACEHOLDER
@@ -421,4 +422,21 @@ class MsPowerpointDocumentBackend(DeclarativeDocumentBackend, PaginatedDocumentB
             for shape in slide.shapes:
                 handle_shapes(shape, parent_slide, slide_ind, doc, slide_size)
 
+            # Handle notes slide
+            if slide.has_notes_slide:
+                notes_slide = slide.notes_slide
+                notes_text = notes_slide.notes_text_frame.text.strip()
+                if notes_text:
+                    bbox = BoundingBox(l=0, t=0, r=0, b=0)
+                    prov = ProvenanceItem(
+                        page_no=slide_ind + 1, charspan=[0, len(notes_text)], bbox=bbox
+                    )
+                    doc.add_text(
+                        label=DocItemLabel.TEXT,
+                        parent=parent_slide,
+                        text=notes_text,
+                        prov=prov,
+                        content_layer=ContentLayer.FURNITURE,
+                    )
+
         return doc