Include instructions for running examples without pixi

[brycemines]

It should be possible to run examples without using pixi now that mantaray is on pypi.

Users are able to pip install the pyproject to get all the example dependencies, but I wonder if we should split up the dependencies or include individual requirements.txt for users who only want to run certain examples.

[milancurcic]

IMO when intended to be used as a dependency in another project, it is indeed advantageous to not also install optional dependencies. So I think it makes sense to split up. But this is a project management thing and won't hold back completing the JOSS review from my end.