However every time I look at the site it seems more and more an abandonware. Only a quickstart and no proper documentation.
The documentation could use a lot of work, but do note that it now lives here:
However, why should I prefer Graphite over, for eg, Munin?
It's still open source but also has a proper documentation.
It doesn't support alarms, right?
You can write a monitoring system that queries Graphite, and send notifications from there. Graphite can give you the raw data that backs the graphs: