WIP: DOC: Add gallery example for magnetic rose

[yvonnefroehlich]

Description of proposed changes

This aims to add a gallery example for adding a magnetic rose to a map using the newly implemented high-level method Figure.pygmt.magnetic_rose.

• Show default behaviour
• Customize appearance via one large rose instead of several small roses for better visibility.
• Label added to declination arrow does not support white spaces (even with using single and double quotation marks).
• [ ]

Preview:

https://pygmt-dev--4381.org.readthedocs.build/en/4381/gallery/embellishments/magnetic_rose.html

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[yvonnefroehlich]

So far, I plot two magnetic roses in this gallery example into one map:

(i) one for the default behavior of the Figure.magnetic_rose method
(ii) one large rose to show different options to adjust the appearance

@GenericMappingTools/pygmt-maintainers, before I continue here, what do you think? Should we go with plotting one large rose or have it like the gallery examples of Figure.scalebar and Figure.directional_rose and plot several small roses? I feel for small magnetic roses the details can be difficult to see.