Gallery example "Decorated lines": Rotation of symbols ("+apangle")

[yvonnefroehlich]

Description of proposed changes

This PR extends the gallery example Decorated lines to show how to rotate the symbols of a decorated line (+apangle).

• Context: Add gallery example "Decorated lines" (style="~") #2564 (comment)
• GMT bug report: Rotating the symbols of a decorated line does not work ("+aangle")  gmt#7591
• GMT bug fix: Fix bug and allow +apangle for decorated lines gmt#8182 -> included in GMT 6.5 🙂
• GMT docs: https://docs.generic-mapping-tools.org/6.5/plot.html#s

Preview: https://pygmt-dev--3016.org.readthedocs.build/en/3016/gallery/lines/decorated_lines.html

Reminders

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• Add tests for new features or tests that would have caught the bug that you're fixing.
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• Write detailed docstrings for all functions/methods.
• If wrapping a new module, open a 'Wrap new GMT module' issue and submit reasonably-sized PRs.
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