you’re right… This is indeed missing in the docs. Thanks for bringing it up. I’ll update the docs.
I suppose the step you’re missing is how to install new plugins:
You can do that wth the app, by enterring System Settings -> Software Updates -> Add or remove packages.
In there you’ll find all the installed and available plugins. Please note that at this point a restart of nymea is required after installing a plugin.
Another option would be to ssh into the system and using apt-get to install plugins as one installs other packages on Raspbian.
Is it that what you’re looking for?