Update first-plugin example to best practices

[psobolewskiPhD]

🧰 Task

Currently we still recommend a setup.cfg and pyproject.toml
Probably best to switch to using just pyproject.toml and maybe a /src dir
https://learn.scientific-python.org/development/

[DragaDoncila]

I agree. I don't want to add a BUNCH of detail about python packaging etc., because we really want folks to focus on the plugin aspects, not the general python aspects. However, now that there is an actual blessed pathway with just pyproject.toml, a src/ dir, and build, I think that should be what we demo.