Improve our 'Creating widgets' section (napari#407)

# References and relevant issues
Follow on from napari#68

# Description

* adds more info on when you would want to choose each type of widget,
clarifying that widgets with access to the native QWidget are equally
extensible.
* clarify you can use `create_widget` in any widget class
* Move the "Avoid imports with forward references" info to plugin best
practices, where we already have a section about avoiding unnecessary
imports
* Move '`magicgui` function widgets as plugin contributions' to be next
to the intro to magicgui function widgets