[feat] Breakout a full avahi set up guide for linux distros that need it

[kn0wmad]

This would belong in Misc Guides, and be linked to from the Linux Root CA page

Notably on PureOS:
The default firewall settings on PureOS are very restrictive, run the following commands

sudo firewall-cmd --permanent --zone=public --add-service=mdns
sudo firewall-cmd --reload