Updated Unifi router instructions

[legoraft]

What does this PR aim to accomplish?:

The Unifi DNS instructions weren't fully up to date anymore in the docs. I updated the IPv4 tutorial and added some notes on VLANs. The IPv6 instructions couldn't be updated, as I currently don't have an IPv6 address in my home router.

How does this PR accomplish the above?:

I added new images and instructions to the Ubiquiti USG section of the docs.

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[yubiuser]

The IPv6 instructions couldn't be updated, as I currently don't have an IPv6 address in my home router.

Even if you don't use IPv6 - are you able to update the screenshots and the instructions? No need to change your local configuration, but it would be nice if we can get rid of the warning about the outdated IPv6 guide...

[yubiuser]

Suggested change

	The default pi-hole behavior (local requests), doesn't work if you have multiple VLANs. You should switch to another [interface](../ftldns/interfaces.md).
	The default Pi-hole interface binding behavior ('Allow only local requests'), doesn't work if you have multiple VLANs. You should switch to another [interface](../ftldns/interfaces.md).

I think it does not work with local request only as all request for VLANs go to the router first. Instead of writing

switch to another [interface]

It might be better to suggest directly "Respond only on interace XXX"

[legoraft]

I'll have a look the coming week, currently not able to check it. It should be possible