Server setup best practices

[Swader]

The server setup documentation should be reviewed for best practices, like not running alertmanager with root and generally making a no-home user for all sensitive operations, as well as closing the unneeded ports etc.

https://wiki.polkadot.network/docs/en/maintain-guides-how-to-monitor-your-node#installing-and-configuring-alertmanager-optional

[salmad3]

#2192 (comment)