Move two pages out of security top level

[wbamberg]

The second part of https://github.com/orgs/mdn/discussions/857, moving:

• https://developer.mozilla.org/en-US/docs/Web/Security/Referer_header:_privacy_and_security_concerns to Web/Privacy. We'll be revisiting the privacy docs in 2026, but it's OK there for now.
• https://developer.mozilla.org/en-US/docs/Web/Security/IFrame_credentialless to Web/HTTP

This leaves us with only the Firefox guide left, which I don't know if we can clean yet.

[github-actions]

Preview URLs

• /en-US/docs/Web/HTTP/Guides/IFrame_credentialless
• /en-US/docs/Web/Privacy/Guides/Referer_header:_privacy_and_security_concerns

Flaws (3)

Note! 1 document with no flaws that don't need to be listed. 🎉

URL: /en-US/docs/Web/HTTP/Guides/IFrame_credentialless
Title: IFrame credentialless
Flaw count: 3

• unknown:
  • No generic content config found
  • no blog root
  • no blog root

[bsmth]

LGTM, thanks 👍🏻