feat: adding an hierarchical document builder,

[davidsbatista]

Related Issues

• implements the 1st part of #7863

Proposed Changes:

Adds a new builder.HierarchicalDocumentBuilder: it's used to split a Document into multiple Document objects of different block sizes building a hierarchical tree structure where each smaller block is a child of a previous larger block.

Some insight of what's being implemented: https://pbs.twimg.com/media/F7ONuajWMAAvuWh?format=jpg&name=4096x4096

How did you test it?

• unit tests
• integration tests
• manual verification

Notes for the reviewer

I'm pushing this into a feature branch to ease the PR review process, after I will push to the same feature branch the code that implements the auto-merge-retriever and then push the feature-branch to the main branch.

Checklist

• I have read the contributors guidelines and the code of conduct
• I have updated the related issue with new insights and changes
• I added unit tests and updated the docstrings
• I've used one of the conventional commit types for my PR title: fix:, feat:, build:, chore:, ci:, docs:, style:, refactor:, perf:, test:.
• I documented my code
• I ran pre-commit hooks and fixed any issue

[anakin87]

• An end-to-end example would be helpful in reviewing this PR. A raw notebook/script showing the whole picture would be sufficient.
• Semantically, this component looks more like a Splitter than a Builder.

[dfokina]

Hey @davidsbatista , could you tag me once the work on the component is done so I can review the docstrings? 🙏

[davidsbatista]

@dfokina I will close this PR soon and instead open it agains the haystack-experimental package - but I can still tag you on the experimental repo