DOC: Add gallery example to show text formatting

[yvonnefroehlich]

Description of proposed changes

Gallery example related to #3976 for formatting text.

Fixes #3976 (comment)

Preview: https://pygmt-dev--3987.org.readthedocs.build/en/3987/gallery/embellishments/text_formatting.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.

Slash Commands

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

• /format: automatically format and lint the code

[michaelgrund]

Suggested change

	superscripts, subscripts, underlining and small caps (for an overview see
	superscripts, subscripts, underlining, and small caps (for an overview, see

[seisman]

Maybe also use formatted text for the plot label and title?

[seisman]

Maybe rephrase it to something like:

There are various options to format text strings added to a plot, e.g.,
text added via Figure.text, title of a plot, labels of colorbars,
Cartesian axes, and legend entries,