gallery and visualisation update

[ojscholten]

Both the gallery section in the docs, and the visualisation section of the user guide, don’t fully cover the capabilities of the gamba.plots module. Updating both of these areas will require the same example code, and would be a nice way to better showcase the plotting capabilities of the library that the API reference alone.

[ojscholten]

The 0.2 refactor, which moved plotting functions into the relevant modules, interrupted this issue, but it's still valid just for each module alone