chore: Remove pydoc-markdown from dev dependencies

[silvanocerza]

Related Issues

Fixed #6119

Proposed Changes:

Remove pydoc-markdown dependency from dev extra.

pydoc-markdown is only used in readme_sync.yml workflow and is also installed via docs/pydoc/requirements.txt ignoring Haystack pyproject.toml.

So it's safe to remove.

This also unpins fastapi version in rest_api.

How did you test it?

There's nothing much to test really.

Notes for the reviewer

N/A

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

[coveralls]

Pull Request Test Coverage Report for Build 6971173188

Warning: This coverage report may be inaccurate.

We've detected an issue with your CI configuration that might affect the accuracy of this pull request's coverage report.
To ensure accuracy in future PRs, please see these guidelines.
A quick fix for this PR: rebase it; your next report should be accurate.

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 21 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.01%) to 40.15%

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Files with Coverage Reduction	New Missed Lines	%
preview/components/converters/pypdf.py	21	0.0%

Totals
Change from base Build 6969071273:	-0.01%
Covered Lines:	10655
Relevant Lines:	26538

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💛 - Coveralls

[anakin87]

🚀