Add instructions to install PyGMT using mamba

[seisman]

Description of proposed changes

Use sphinx-design's tab-set directive to show conda and mamba usage.

Fixes #1663.

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 commands are:

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

[seisman]

I'm not sure if this is a good way to document how to install PyGMT using mamba. Please let me know what you think.

[weiji14]

I'm not sure if this is a good way to document how to install PyGMT using mamba. Please let me know what you think.

Looks great actually! Much cleaner than repeating in two lines.

Note that the sync directive only works with javascript I think, but even without javascript, the user can still switch between the conda and mamba tabs. The tab switching also works with a keyboard's left and right arrows, so seems ok accessibility-wise.