Tighten documentation quality: restore KIE training snippet and fix docstring rendering in visualization

[Copilot]

This updates documentation quality in two focused areas: it restores missing user guidance in custom model training docs and removes a docstring formatting issue that caused Sphinx/docutils rendering warnings. The goal is to keep feature docs complete while improving generated documentation output without expanding KIE scope.

• Docs completeness: custom model training

  • Restored the removed KIE detection model loading snippet in docs/source/using_doctr/custom_models_training.rst.
  • Keeps KIE coverage at existing scope (no new KIE feature docs added), while preserving a previously documented loading path.

• Docstring rendering correctness

  • Updated doctr/utils/visualization.py (draw_boxes) docstring to avoid reStructuredText parsing issues in generated docs.
  • Adjusted punctuation and inline formatting so the API docs render cleanly.

def draw_boxes(...):
    """Draw an array of relative straight boxes on an image.

    Args:
        boxes: array of relative boxes, of shape ``(*, 4)``
        ...
    """