Figure.image: Migrate the 'monochrome' parameter to the new alias system

[seisman]

Preview: https://pygmt-dev--4088.org.readthedocs.build/en/4088/api/generated/pygmt.Figure.image.html

Upstream docs: https://docs.generic-mapping-tools.org/6.5/image.html

[Copilot]

Pull Request Overview

This PR migrates the 'monochrome' parameter from the old alias system to the new alias system in the Figure.image method.

• Removes 'monochrome' from the @use_alias decorator and implements it through the new AliasSystem
• Updates documentation to improve clarity and formatting
• Adds the monochrome parameter explicitly to the function signature

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