Further clarify use of qt backend in Step 4 of setting up a development environment

[willingc]

🐛 Bug

For document at https://napari.org/dev/developers/contributing/dev_install.html#setting-up-a-development-installation Step 4 is a bit confusing for someone new to the Qt ecosystem.

In Step 4,

Perhaps make it more explicit that the contributor chooses one of the options with some further guidance to why. Or provide a sensible default for most.