Add sphinx-copybutton extension to easily copy codes

[seisman]

Description of proposed changes

Try it:

• Go to https://pygmt-git-sphinx-copybutton.gmt.vercel.app/tutorials/inset.html, click the copy button on the top right corner of the code block, and copy it to your editor
• Go to https://pygmt-git-sphinx-copybutton.gmt.now.sh/api/generated/pygmt.Figure.inset.html#pygmt.Figure.inset the example section, click the copy button, and copy it to your editor. The prompt (>>> and ...) and comments are trimmed.

Reference: https://sphinx-copybutton.readthedocs.io/en/latest/

Fixes #833

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

[weiji14]

The copybutton looks a bit small but hopefully people can notice it:

[liamtoney]

Great addition!