Make it easier to convert an existing phoenix app into a desktop app

[dominicletz]

This is feedback I've received from a user. Right now the steps to convert an existing phoenix app are undocumented and potentially difficult.

We should probably document the process and also reduce friction. Some initial thoughts:

• The explicit usage of Desktop.Endpoint might not be needed (anymore) with more recent phoenix apps
• The explicit Desktop.Window creation could benefit from a simpler default.

A write-up of the minimal steps is probably a good first step here.