Add an introductory PyGMT tutorial

I think there should be a PyGMT tutorial, much like the [GMT tutorial](https://docs.generic-mapping-tools.org/6.1/tutorial.html). I don't think it requires as much explanation as what's happening under the hood, but would ideally recreate the Laboratory Exercises. The PyGMT User Guides cover a variety of topics, but I think following the GMT tutorial gives us a good roadmap on what features to explain for someone trying to learn.

Proposed structure:
- [x] Tutorial introduction (#1607)
- [x] Create a basic map figure using `coast` (#1607)
- [ ] Plot data on a map (#2126)
- [ ] Create a contour map (#2126)
- [ ] Add a color pallet to a map (#2126)
- [ ] Create a 3D map