DOC: Add basic tutorial "Plotting polygons"

[yvonnefroehlich]

Description of proposed changes

Add a basic tutorial for plotting polygons, explaining the pen, fill, and close parameters.

Related basic tutorials:

• Plotting lines: https://www.pygmt.org/dev/tutorials/basics/lines.html
• Plotting text: https://www.pygmt.org/dev/tutorials/basics/text.html
• Plotting symbols: DOC: Add basic tutorial "Plotting single-parameter symbols" #3598

Fixes #3591

Preview: https://pygmt-dev--3593.org.readthedocs.build/en/3593/tutorials/basics/polygons.html

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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[seisman]

A little off-topic: what do you think about updating the existing "Plotting lines" tutorial to make it more eye-catching? Currently, the tutorial just plots lines with two or more data points. I feel a more complicated line/curve like sine/cosine functions are better.

[yvonnefroehlich]

A little off-topic: what do you think about updating the existing "Plotting lines" tutorial to make it more eye-catching? Currently, the tutorial just plots lines with two or more data points. I feel a more complicated line/curve like sine/cosine functions are better.

Will look and work on this after AGU.