[Feature] Support defining reading order from the layout stage

### Requested feature
Some layout models, such as [PP-DocLayoutV3](https://huggingface.co/PaddlePaddle/PP-DocLayoutV3), include reading order in their output. Ideally, Docling could support using that order instead of computing it again later in the reading order stage.

Related: #3224, #3233

Start by reviewing related issues #3224 and #3233, then trace the layout stage and the later reading order stage. Done means a layout model's supplied reading order can be used instead of recomputing it, while existing behavior remains supported.

Written by the indexing model from the issue text.