fix: Remove Extra Space Before and After Group Items

[RamanaMenda]

Removing extra space before and after group items to resolve the issue raised in #2745

Resolves #371
Resolves docling-project/docling#2745

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[RamanaMenda]

#371

[ceberam]

@RamanaMenda thanks for suggesting a fix for this issue
Please, make sure to resolve all the code checks before pushing new commits. I strongly recommend installing pre-commit in your local repository. It will run the code checks every time you execute git commit. Simply type uv run pre-commit install on your local repository.
In addition, keep in mind that fixing the serialization in docling-core will have an impact on docling library. Some backend parsers may trim trailing blanks in formatted text and, after this PR, the serialization may lead to missing blank space between words. In addition, some ground-truth files in the tests folder contain the (wrong) extra spaces and will need to be updated to ensure that the regression tests pass.
Therefore, I see the following pipeline to fix this issue:

1. Fix this issue of extra spaces in docling-core and submit a PR.
2. Get the PR approved, merged, and published in a new release of docling-core.
3. Fix backend parsers and ground truth files in docling to ensure that necessary blank spaces are not removed.
4. Pin the new release version of docling-core in docling's pyproject.toml file.
5. Update the uv.lock file (e.g., with uv lock --upgrade-package docling-core).
6. Ensure all tests pass in docling with the new release of docling-core.
7. Submit a PR in docling project with the changes above.