Update subdomains-outside-cloudflare.mdx

[kbitr]

Summary

adding a note for the reverse; subdomain setups after customers told they were unable to find it. please refine wording as you like

• kai

Screenshots (optional)

n/a

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• If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.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.

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