Description of proposed changes

Following https://docs.generic-mapping-tools.org/dev/text.html#description, this PR addes a table to the Technical References for formatting text.

Preview: https://pygmt-dev--3976.org.readthedocs.build/en/3976/techref/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