fix(markdown): Setext heading support (#2359)

Signed-off-by: Matvei Smirnov <[email protected]>
Co-authored-by: Matvei Smirnov <[email protected]>

Author: Vdaleke

docling/backend/md_backend.py

@@ -249,7 +249,10 @@ def _iterate_elements(  # noqa: C901
 
         # Iterates over all elements in the AST
         # Check for different element types and process relevant details
-        if isinstance(element, marko.block.Heading) and len(element.children) > 0:
+        if (
+            isinstance(element, marko.block.Heading)
+            or isinstance(element, marko.block.SetextHeading)
+        ) and len(element.children) > 0:
             self._close_table(doc)
             _log.debug(
                 f" - Heading level {element.level}, content: {element.children[0].children}"  # type: ignore

tests/data/groundtruth/docling_v2/blocks.md.md

@@ -16,6 +16,14 @@ Heading:
 
 Empty heading:
 
+Setext heading level 1:
+
+# my setext heading level 1
+
+Setext heading level 2:
+
+## my setext heading level 2
+
 Indented code block:
 
 ```

tests/data/md/blocks.md

@@ -22,6 +22,16 @@ Empty heading:
 
 #
 
+Setext heading level 1:
+
+my setext heading level 1
+=========================
+
+Setext heading level 2:
+
+my setext heading level 2
+-------------------------
+
 Indented code block:
 
     print("Hi!")