Add an example for Oblique Mercator projection

[maxrjones]

Description of the desired feature

Add an example to the projections section of the PyGMT user guide showing in the three ways to define the Oblique Mercator projection.

References:

As always, the contributing guide: https://github.com/GenericMappingTools/pygmt/blob/master/CONTRIBUTING.md
GMT documentation for the Oblique Mercator projection: https://docs.generic-mapping-tools.org/dev/cookbook/map-projections.html#jo
The other PyGMT projection examples (located in examples/projections/).

Are you willing to help implement and maintain this feature? I would welcome help, but can add this for v0.4.0 if no one else wants to.

[eemcmullan]

Hello! I am interested in contributing to this project, and would be happy to work on this issue.

[maxrjones]

Thanks, @eemcmullan! Feel free to open a PR and reach out if you have any questions!