Reorganize the documentation

Will close issue #257 and #273 
We need a little wrap-up:

>
You're done! Now create a form with an `imageFile` field that uses the `upload` type.
When you submit and save, the uploaded file will automatically be moved to the
location you configured and the `imageName` field will be set to the filename of the
uploaded file.

... we could even show an example of the form.