Copy `make_napari_viewer` docstring info into docs

[lucyleeow]

🧰 Task

The make_napari_viewer docstring includes a lot of useful info that is not included in the testing section in the docs. See: https://napari.org/dev/developers/contributing/testing.html#testing-with-qt-and-napari-viewer
We do mention make_napari_viewer but a lot of useful information from the docstring is not included.

I could just copy it across but to ease future maintenance, maybe we could do something more clever? Maybe whatever magic happening with the plugins contribution (relevant ref: napari/npe2#340 (comment))

cc @psobolewskiPhD @melissawm ?

[psobolewskiPhD]

Related: #150

[psobolewskiPhD]

The info about the fixture would be helpful to plugin devs doing testing too, see: #373

[lucyleeow]

Argh I missed that one. I'm going to close this and comment on the original issue, thanks @psobolewskiPhD !