Add notice that the onion dns server has been offline for a while

[LenaWil]

Summary

Okay, this probably shouldn’t be in documentation, but there wasn’t a good way to contact Cloudflare since I don’t have premium support and technically the documentation is incorrect right now. 🙃

Related to #23042.

Screenshots (optional)

Documentation checklist

• The documentation style guide has been adhered to.
• If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.
• Files which have changed name or location have been allocated redirects.

[hyperlint-ai]

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PR Change Summary

Added a notice regarding the offline status of the onion DNS server in the documentation.

• Updated documentation to inform users that the onion DNS server has been offline for several weeks.
• Clarified that the hidden resolver is still experimental and should not be used in production.

Modified Files

• src/content/docs/1.1.1.1/other-ways-to-use-1.1.1.1/dns-over-tor.mdx

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[RebeccaTamachiro]

Following up in #23042