Fix missing PyGMT images in user guide

[santisoler]

Use fig.show(method="notebook") in user guide pages that use PyGMT to generate plots. This way jupyter-sphinx is able to render the images in the generated HTML files.

Relevant issues/PRs:

Fixes #473

[santisoler]

I totally agree. That's a much better solution than this one. I'll revert the previous changes and then apply the one you suggested.