DOC: Plasticity documentation update

[pgrigorev]

This is a merge request to answer comments in the JOSS review

[pgrigorev]

I made the updates to address the issues in the JOSS review:

• Added Installation and testing section in plasticity documentation explaining why most of the tests are skipped.
• Added examples how to create dislocations in FCC and Diamond Cubic structures.

On top of that I improved the the interactive visualisation to colour the atoms according to crystallographic structures identified by CNA from OVITO. I also managed to modify the info displayed on mouse hover to make it more meaningful. At the moment it is in the file in the docs/applications folder. If this is useful I could put it in matscipy.visualise.py for example.

It seems a bit confusing to try to find documentation for matscipy. If I google matscipy documentation together with updated page on https://libatoms.github.io/matscipy/ I get outdated https://libatoms.github.io/matscipy/matscipy.html and also something on readthedocs: https://matscipy.readthedocs.io/en/latest/. Could we get rid of outdated pages?

[jameskermode]

Thank you! The failing CI should be fixed by #198 and is unrelated to changes here so I'll go ahead and merge this now so that #193 can progress.