WIP: Add oblique Mercator projection gallery examples

[willschlitzer]

This PR adds separate gallery examples for the three oblique Mercator projections.

Fixes #

Reminders

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst.
• Write detailed docstrings for all functions/methods.
• If adding new functionality, add an example to docstrings or tutorials.

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[weiji14]

Err, I think someone wanted to work on #1073 and has started a branch at https://github.com/eemcmullan/pygmt/tree/docs-add already, do you want to maybe combine efforts?

[eemcmullan]

@willschlitzer I wouldn't mind combining efforts. I could add in the descriptions for each projection?

[willschlitzer]

It looks like I didn't do my homework properly before starting this; it was @eemcmullan's forum post that reminded me that I had never made oblique Mercator gallery examples. Seems like #1251 is further along than this PR, so I'll close this one and add comments over there!