Add figures to examples of the docs

In the section examples of the modules docs, I think we can add the figure generated by each example. I think it would help to understand them and also would make the docs more friendly-user (at least for me).

For example in the [mask docs](https://docs.generic-mapping-tools.org/dev/mask.html) it says:
```
gmt begin mask
gmt math -T-90/90/10 -N2/1 0 = | gmt mask -Gyellow -I30m -R-75/75/-90/90 -JQ0/7i \
-S5d -T -Bafg10 -BWSne+t"Mask for points with r = 5 degrees"
gmt end show

```
and we could add the figure generated by these commands:


**Are you willing to help making the figures and add them in the docs?** Yes. Also maybe Meghan could do it?