Add one-liner examples to the docs

[maxrjones]

As suggested in #4, this feature request is to add a simple example or two for each remote dataset, similar to those in the GMT man pages.

[Esteban82]

Is this NOT cover by the usage section (copy below)? Or the idea is to have a more detailed example?

You access the GEBCO global relief grid by specifying the special name

@earth_gebco[_rru[_reg]]

[seisman]

I think the idea is a command like

gmt grdimage @earth_relief_01d -pdf map