AsciiDoc backend fails parsing admonitions

[jramcast]

Bug

Asciidoc backend cannot parse files that contain admonitions in block syntax.

Steps to reproduce

1. Create the test.adoc file with these contents:

= Title 1

This is regular content.

== Title 2

More content

[NOTE]
====
Content displayed as an admonition
====

2. Run this code:

from docling.document_converter import DocumentConverter

source = "test.adoc"
converter = DocumentConverter()
result = converter.convert(source)

3. The generated error is an AttributeError:

Traceback (most recent call last):
  File "/home/student/dev/test-docling/main.py", line 14, in <module>
    main()
  File "/home/student/dev/test-docling/main.py", line 7, in main
    result = converter.convert(source)
  File "/home/student/dev/test-docling/.venv/lib/python3.10/site-packages/pydantic/_internal/_validate_call.py", line 38, in wrapper_function
    return wrapper(*args, **kwargs)
  File "/home/student/dev/test-docling/.venv/lib/python3.10/site-packages/pydantic/_internal/_validate_call.py", line 111, in __call__
    res = self.__pydantic_validator__.validate_python(pydantic_core.ArgsKwargs(args, kwargs))
  File "/home/student/dev/test-docling/.venv/lib/python3.10/site-packages/docling/document_converter.py", line 172, in convert
    return next(all_res)
  File "/home/student/dev/test-docling/.venv/lib/python3.10/site-packages/docling/document_converter.py", line 193, in convert_all
    for conv_res in conv_res_iter:
  File "/home/student/dev/test-docling/.venv/lib/python3.10/site-packages/docling/document_converter.py", line 228, in _convert
    for item in map(
  File "/home/student/dev/test-docling/.venv/lib/python3.10/site-packages/docling/document_converter.py", line 269, in _process_document
    conv_res = self._execute_pipeline(in_doc, raises_on_error=raises_on_error)
  File "/home/student/dev/test-docling/.venv/lib/python3.10/site-packages/docling/document_converter.py", line 292, in _execute_pipeline
    conv_res = pipeline.execute(in_doc, raises_on_error=raises_on_error)
  File "/home/student/dev/test-docling/.venv/lib/python3.10/site-packages/docling/pipeline/base_pipeline.py", line 52, in execute
    raise e
  File "/home/student/dev/test-docling/.venv/lib/python3.10/site-packages/docling/pipeline/base_pipeline.py", line 44, in execute
    conv_res = self._build_document(conv_res)
  File "/home/student/dev/test-docling/.venv/lib/python3.10/site-packages/docling/pipeline/simple_pipeline.py", line 41, in _build_document
    conv_res.document = conv_res.input._backend.convert()
  File "/home/student/dev/test-docling/.venv/lib/python3.10/site-packages/docling/backend/asciidoc_backend.py", line 75, in convert
    doc = self._parse(doc)
  File "/home/student/dev/test-docling/.venv/lib/python3.10/site-packages/docling/backend/asciidoc_backend.py", line 117, in _parse
    item = self._parse_section_header(line)
  File "/home/student/dev/test-docling/.venv/lib/python3.10/site-packages/docling/backend/asciidoc_backend.py", line 303, in _parse_section_header
    marker = match.group(1)  # The list marker (e.g., "*", "-", "1.")
AttributeError: 'NoneType' object has no attribute 'group'

It appears that the error is caused by the _is_section_header method in the asciidoc_backend.
This method is considering the ==== line as a title, when in fact the line is part of the admonition block syntax.

Docling version

Docling version: 2.10.0
Docling Core version: 2.9.0
Docling IBM Models version: 2.0.7
Docling Parse version: 3.0.0

Python version

Python 3.10.15