Improve 'getting started' experience on Flathub

We can improve the experience for PD newcomers. Currently we show a screenshot of a random PD patch and in the description we say:

* this is a fork of Pure Data
* it's experimental and may have issues.
* Pure Data is portable, and has a complex history
* some of the things Pure Data can do

Possible improvements:

* show more self-explanatory and perhaps more eye-catching screenshots
* summarize what PD can do in the 1st paragraph, not the last
* provide links or instructions for finding ready-to-use examples of Pd patches (we should bundle some with the Flatpak, and make them easy to discover)
* provide links or instructions for finding getting started tutorials for Pd (these should be included in the Flatpak and easy to discover)