Updated docstrings in pymc.step_methods.metropolis for BinaryGibbsMetropolis to follow numpydoc

[HangenYuu]

Description

First time contributor to open-source and the project for me. I updated the docstring of BinaryGibbsMetropolis to follow numpydoc format. I also updated the Metropolis init while trying to check how Sphinx works (deleting it would be pointless so I keep it there).

Related Issue

• [ ] Closes #
• [x] Related to #5459

Checklist

• [x] Checked that the pre-commit linting/style checks pass
• [x] Included tests that prove the fix is effective or that the new feature works
• [x] Added necessary documentation (docstrings and/or example notebooks)
• [ ] If you are a pro: each commit corresponds to a relevant logical change

Type of change

• [ ] New feature / enhancement
• [ ] Bug fix
• [x] Documentation
• [ ] Maintenance
• [ ] Other (please specify):

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📚 Documentation preview 📚: https://pymc--7212.org.readthedocs.build/en/7212/

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