Provide a gallery example to show coloring of points by categories

[michaelgrund]

To close the >2 year old issue #244 here's a gallery example highlighting the coloring of points by categories.

Reminders

• [] Run make format and make check to make sure the code follows the style guide.
• [] 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