fix: improve HTML layer detection, various MD fixes (#1241)

Markdown fixes:
- properly propagate section header levels
- improve handling of list subroots without text

Signed-off-by: Panos Vagenas <pva@zurich.ibm.com>

tests/data/groundtruth/docling_v2/wiki_duck.html.md

@@ -1,3 +1,26 @@
+## Contents
+
+- (Top)
+- 1 Etymology
+- 2 Taxonomy
+- 3 Morphology
+- 4 Distribution and habitat
+- 5 Behaviour Toggle Behaviour subsection
+    - 5.1 Feeding
+    - 5.2 Breeding
+    - 5.3 Communication
+    - 5.4 Predators
+- 6 Relationship with humans Toggle Relationship with humans subsection
+    - 6.1 Hunting
+    - 6.2 Domestication
+    - 6.3 Heraldry
+    - 6.4 Cultural references
+- 7 See also
+- 8 Notes Toggle Notes subsection
+    - 8.1 Citations
+    - 8.2 Sources
+- 9 External links
+
 # Duck
 
 - Acèh