WIP: Add gallery example showing how to fill the area between two curves

[yvonnefroehlich]

Description of proposed changes

This PR is converted to draft as a high-level method is planned for fill the area between two curves

-M of plot allows to fill the area between two curves with colors or patterns. This PR addes a gallery example showing how to do this:

• PR WIP: Figure.plot: Add alias "fillcurves" for "M" #3169
• Add code for gallery example
• Add documentation and comments

Preview: https://pygmt-dev--3168.org.readthedocs.build/en/3168/gallery/lines/curves_area.html

Fixes #

Reminders

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst.
• Write detailed docstrings for all functions/methods.
• If wrapping a new module, open a 'Wrap new GMT module' issue and submit reasonably-sized PRs.
• If adding new functionality, add an example to docstrings or tutorials.
• Use underscores (not hyphens) in names of Python files and directories.

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[yvonnefroehlich]

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