Splitting of the projections gallery into projection types and complete the examples for azimuthal projections

[phloose]

This pull request adds sub sections in the projection examples gallery for the five main projection groups that GMT supports and completes the azimuthal projection examples.

Related to #292

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• If adding new functionality, add an example to docstrings or tutorials.

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[leouieda]

@xibalba01 thanks for doing all this work! I made a few minor corrections to the text. Please see if you agree and after they're all addressed we can merge this in.

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[leouieda]

Thanks @xibalba01! This was great. Looking forward to new contributions in the future 👍

[phloose]

Glad i could help a bit @leouieda! Looking forward, too!