feat: MarkdownHeaderSplitter

[OGuggenbuehl]

Proposed Changes:

Implement MarkdownHeaderSplitter to split Documents written in .md based on their headers

How did you test it?

unit tests

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: and added ! in case the PR includes breaking changes.
• I documented my code
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[CLAassistant]

All committers have signed the CLA.

[sjrl]

@OGuggenbuehl definitely looks like an interesting approach! I've left an initial set of comments, but to further review I'd appreciate if you could add a set of tests like the ones we have for the DocumentSplitter https://github.com/deepset-ai/haystack/blob/main/test/components/preprocessors/test_document_splitter.py

This will help me be able to review the actual algorithm for splitting since it's easier to understand with examples.

[sjrl]

Thanks for your continued work on this @OGuggenbuehl!

Some general comments. Could you:

• Add a release note for this PR following the instructions here
• Could you make sure to include our license header to the beginning of each file you've added. You can find an example of the license header here
• Please make sure to sign the CLA agreement (docs about it here) from this comment
• If you haven't already please also set up pre-commit hooks using pre-commit install. You can find more info about that in this section of our contribution guidelines.
• Also in the future feel free to open branches directly in Haystack instead of using a fork. This makes it slightly easier to pull down your code to review locally.

[coveralls]

Pull Request Test Coverage Report for Build 19575164151

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.03%) to 91.467%

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Totals
Change from base Build 19573565148:	0.03%
Covered Lines:	13860
Relevant Lines:	15153

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💛 - Coveralls

[OGuggenbuehl]

@sjrl mypy has started complaining about a file that is unrelated to this pr:

haystack/tracing/datadog.py:34: error: Argument 2 to "set_tag" of "Span" has incompatible type "Union[str, int, float]"; expected "Optional[str]"  [arg-type]

I assume this is an upstream issue and I can simply rebase after it's been fixed upstream?