Spacy-layout

- Add `as_tuples` argument in `spaCyLayout.pipe`, analogous to spaCy's `Language.pipe`. (36)
- Add support for document index tables. (34, mkessy)
- Fix typos in docs. (33, svlandeg)
- Add code snippet for visualizing a page with `matplotlib` to docs. (26, wjbmattingly)
- Add [serialization](https://github.com/explosion/spacy-layout#serialization) section to docs with note on deserializing with extension attributes.

- Fix regression that would cause incorrect pagination numbers to be represented.

- Allow `DoclingDocument` as input to `spaCyLayout.__call__` to convert already processed documents to spaCy `Doc` objects.

- Add `Doc._.markdown` with Markdown representation of the document.

- Fix serialization of extension attributes and `pandas.DataFrame` via spaCy's `DocBin`. (11, 14)

- Fix bounding boxes for top left origin, refactor and add tests.