Docs for PA trigger module

[clark2668]

The only real documentation on the PA trigger module is from Dan Smith in Dec 2020 (https://uppsala.app.box.com/s/1crnmq2jgnt5e6xs3d1zzntfq4pkwadm), and it's kinda hard to find. It might be nice to find a permanent home for the documentation, or copy it into the NuRadioReco docs somehow.