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.

---

By submitting this pull request, I confirm the following:

1. I have read and understood the contributors guide, as well as this entire template. I understand which branch to base my commits and Pull Requests against.
2. I have commented my proposed changes within the code and I have tested my changes.
3. I am willing to help maintain this change if there are issues with it later.
4. It is compatible with the EUPL 1.2 license
5. I have squashed any insignificant commits. (git rebase)
6. I have checked that another pull request for this purpose does not exist.
7. I have considered, and confirmed that this submission will be valuable to others.
8. I accept that this submission may not be used, and the pull request closed at the will of the maintainer.
9. I give this submission freely, and claim no ownership to its content.

---

• I have read the above and my PR is ready for review. Check this box to confirm

[netlify]

✅ Deploy Preview for pihole-docs ready!

Name	Link
🔨 Latest commit	3aafdbc
🔍 Latest deploy log	https://app.netlify.com/projects/pihole-docs/deploys/6867f0a4c078020008e2b1fc
😎 Deploy Preview	https://deploy-preview-1259--pihole-docs.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify project configuration.