fix: correct extract_items_range method for rich tables

[capoan]

The purpose of this PR is to solve issue #427 (rich table items extraction issue).

A way to extract rich table items from a DoclingDocument is given in #427 by copying the items as-is and then using _DocIndex.index. Here, we propose another solution:

• We add a "root" parameter to _DocIndex.index to extract only this "root" element (and all his descendants) from a document.
• We use this new "root" parameter of _DocIndex.index in the DoclingDocument method extract_items_range to support extracting a range of items, including rich table items.

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