Various edits and tweaks for clarity

Went around trying to do some cleanup...

My :::note in "Sign Certificates with Public Keys" guide is really a warning, so I switched it to :::caution. It could even be considered danger, but I want to avoid overuse of red and we use private key copying in examples elsewhere.
I changed a :::danger in unsafe_routes docs to :::caution. I think the worst that happens here is you tear your hair out a little bit during setup, so I wouldn't consider this dangerous per se.
"EXPERIMENTAL: asdf" looked a little jarring to me on the relays page, so I reworded it. I also want to encourage people to use it as it's a very useful tool. I changed it from :::note to :::caution because we use caution for other experimental notes (e.g. DNS.)
Clarified ciphers used and recommended AES.
Explained how relays.relays works a little more as it's "backwards" of what most seem to expect.
Moved things around a little bit in the DNS guide to try to get to the meat of it quicker, and moved limitations further down the page (also fleshed them out a bit more.) The DNS hostname validity tool now lives under the Limitations section.

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