Security considerations section links to non-versioned GH doc

[pes10k]

This issue is being filed as part of the requested PING privacy review #106

Currently the Security considerations section links to https://github.com/w3c/window-placement/blob/main/security_and_privacy.md for "more details for additional explorations of security concerns". It would be better to not link to non-versioned documents maintained by the WG in this manner if its not necessary. I suggest either inlining the parts of security_and_privacy.md that the authors think are relevant, or removing the link all together