Improve 'getting started' experience on Flathub

[ssssam]

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)

[ssssam]

One simple improvement would be to add a bookmark in the Gtk filechooser to the /app/lib/pd-l2ork/doc folder. These examples are very useful but there doesn't seem to be a way to access them from the menus.