Add OpenWrt router guide

[ihatetoregister]

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What does this PR aim to accomplish?:

Add OpenWrt router guide

How does this PR accomplish the above?:

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Link documentation PRs if any are needed to support this PR:

N/A

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[yubiuser]

Tests are failing.

And you need to include the new file in the table of contents in mkdocs.yml

[yubiuser]

Suggested change

Here's a short guide to configure Pi-hole as the DNS server for your OpenWrt router.

[yubiuser]

I think this whole section is not needed. It should be clear that a guide for OpenWrt & Pi-hole requires an OpenWrt router and Pi-hole....

[yubiuser]

I think this image can be cropped. The relevant section is only in the upper quarter of it.

[ihatetoregister]

sorry not waisting any more time on this, take it or leave it...

[yubiuser]

Ok, thanks.