Add mDNS network discovery docs

[MichaIng]

E.g. for DietPi-Dashboard: ravenclaw900/DietPi-Dashboard#170
But actually, while I personally thought that a server admin web interface should probably not broadcast itself, this may be handy in many other cases. We could probably add a generic docs tab to all web UI software, or even all software with network interfaces or APIs, via Avahi daemon. As a next stage we could make the generation of these configs an option in dietpi-software itself, to be not too intrusive with additional prompts, via dietpi.txt option at least?

[StephanStS]

@MichaIng: What could we add at which docu section covering this issue?
Recently, we added the Avahi section: Description for Avahi-Daemon added to Advanced Networking #1049.

[MichaIng]

Maybe we can add a little "tip" box to every web interface/access tab, like

You can make your DietPi/Home Assistant accessible via <hostname>.local (default: DietPi.local), by installing Avahi-Daemon. See our Avahi-Daemon docs for details."

Um, we really need to make use of includes, else we just have so many code/text duplication for all of this: #402