Plugins docs reorg

[melissawm]

🐛 Bug

We should reevaluate the organization of the plugin docs, especially the content that was added as part of workshop material (e.g. https://napari.org/dev/plugins/virtual_environment_docs/index.html and https://napari.org/dev/plugins/testing_workshop_docs/index.html)

Additional context

We talked about this in the docs working group meeting on Feb 27, 2025.