Add a gallery example showing the usage of text symbols

[michaelgrund]

Description of proposed changes

This PR adds a gallery example that shows the usage of text symbols via Figure.plot().

Preview at https://pygmt-git-gallery-text-symbols-gmt.vercel.app/gallery/symbols/text_symbols.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 adding new functionality, add an example to docstrings or tutorials.

Slash Commands

You can write slash commands (/command) in the first line of a comment to perform
specific operations. Supported slash commands are:

• /format: automatically format and lint the code
• /test-gmt-dev: run full tests on the latest GMT development version

[seisman]

Is "text symbols" different from normal texts? Why not using the Figure.text()?

[michaelgrund]

Is "text symbols" different from normal texts? Why not using the Figure.text()?

I expanded the docstring and added a further sentence that is used in the GMT docs explaining why to use text symbols. In my opinion we should at least mention that it's possible to plot text symbols also via Figure.plot() not only using Figure.text().

[seisman]

The example looks good to me except a few suggestions.

[weiji14]

Had an idea to spell out 'πgmt', but ok with this example too 😆

[michaelgrund]

Had an idea to spell out 'πgmt', but ok with this example too 😆

Nice idea but I think it's not possible to mix Symbols and standard text here.