docs: add guide to use DNS-over-HTTPS with `AdguardTeam/dnsproxy`

[cvscarlos]

What does this PR aim to accomplish?:

This PR creates a guide that offers an alternative way to enable DNS-over-HTTPS on your Pi-Hole.

I spent a few hours trying to get cloudflared working, unfortunately without success. So dnsproxy solved my problems, just as it helped me I hope it can help others through this guide to save hours of trial and error.

How does this PR accomplish the above?:

Add a page under the "Guide" section explaining how to use dnsproxy to configure DNS-over-HTTPS for Pi-Hole.

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