Add --output CLI argument to export changelog to Markdown (#2019)

[Mohima6]

Simplifies release workflows by avoiding manual copy-paste. Keeps backward compatibility: if no --output is provided, the changelog prints to stdout. Complements recent Markdown updates to the changelog generator (#2019)

Reference Issues/PRs

Closes #2019

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What does this implement/fix?

This PR adds an optional CLI argument `--output` to the changelog generator.

- When `--output <file>` is provided, the generated changelog is written to the specified Markdown file.
- If no output file is provided, the changelog is printed to stdout, keeping the previous behavior.
- Captures both the rendered changelog and contributor list.

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What should a reviewer concentrate their feedback on?

- Correct handling of CLI argument parsing.
- Proper writing of the Markdown file while retaining the printed output.
- Backward compatibility: ensure running without `--output` behaves as before.

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Did you add any tests for the change?

Manual testing can be done by running:
- `python build_tools/changelog.py` (should print to terminal)
- `python build_tools/changelog.py --output CHANGELOG.md` (should create/update file)

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Any other comments?

- This PR complements the previous Markdown changelog updates (#2016).
- Optional: future improvements could include simultaneous printing and file writing.

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PR checklist

• The PR title starts with either [ENH], [MNT], [DOC], or [BUG]. ✅ Suggested title: [ENH] Add --output CLI argument to export changelog to Markdown (#2019)
• Added/modified tests
• Used pre-commit hooks when committing to ensure that code is compliant with hooks

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