Update docs on visualisation and post processing

[KAClough]

Review comment: The documentation links to this page for post-processing with yt. But there's no explanation on how to use these scripts. Similarly for visit. The included scripts have no comments. E.g., what is the difference between PlotSlice.py and PlotSliceRemix.py? How to use and modify these scripts?

Miren: I agree that the documentation and comments for the postprocessing tools is not up to the same standard of this code and could be improved. However, these tools are not part of this repository and should therefore not be considered an integral part of the main code. We link to them merely as examples and they are provided "as is". For your specific example of the difference between PlotSlice.py and PlotSliceRemix.py, please refer to the comments at the top of the latter where you will also find usage instructions.

Katy: It would be nice to have a little more guidance on these scripts at some point :-) Maybe we can make and link to a video from our next online training programme.