Add the sphinx-copybutton extension for easier copying codes

[seisman]

Description of the desired feature

The examples in the docstring have Python prompt likes >>> and ..., which makes it difficult to copy. For example, https://www.pygmt.org/dev/api/generated/pygmt.datasets.load_earth_relief.html#pygmt.datasets.load_earth_relief.

Directly copy the codes gives me the following codes:

>>> # load the default grid (pixel-registered 01d grid)
>>> grid = load_earth_relief()
>>> # load the 30m grid with "gridline" registration
>>> grid = load_earth_relief("30m", registration="gridline")
>>> # load high-resolution grid for a specific region
>>> grid = load_earth_relief(
...     "05m", region=[120, 160, 30, 60], registration="gridline"
... )

which is not ready for use.

The sphinx-copybutton extension can add a copy button to code blocks so readers can copy the codes by clicking the button. It can also strip the prompts.

I think the extension would make the documentation easier to use. What do you think @GenericMappingTools/python.

Are you willing to help implement and maintain this feature? Yes