fix: fix header levels for DOCX & HTML (#184)

Signed-off-by: Panos Vagenas <35837085+vagenas@users.noreply.github.com>

tests/test_backend_html.py

@@ -0,0 +1,30 @@
+from pathlib import Path
+
+from docling.backend.html_backend import HTMLDocumentBackend
+from docling.datamodel.base_models import InputFormat
+from docling.datamodel.document import InputDocument, SectionHeaderItem
+
+
+def test_heading_levels():
+    in_path = Path("tests/data/wiki_duck.html")
+    in_doc = InputDocument(
+        path_or_stream=in_path,
+        format=InputFormat.HTML,
+        backend=HTMLDocumentBackend,
+    )
+    backend = HTMLDocumentBackend(
+        in_doc=in_doc,
+        path_or_stream=in_path,
+    )
+    doc = backend.convert()
+
+    found_lvl_2 = found_lvl_3 = False
+    for item, _ in doc.iterate_items():
+        if isinstance(item, SectionHeaderItem):
+            if item.text == "Etymology":
+                found_lvl_2 = True
+                assert item.level == 2
+            elif item.text == "Feeding":
+                found_lvl_3 = True
+                assert item.level == 3
+    assert found_lvl_2 and found_lvl_3