Revisit Pulumi Cloud security whitepaper (#20586)

* Revisit Pulumi Cloud security whitepaper to keep it up to date

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* Mention MFA/TOTP for password-based accounts in security whitepaper

* Update content/security/pulumi-cloud-security-whitepaper.md

Co-authored-by: Derek <dschaller@users.noreply.github.com>

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Co-authored-by: Claude Fable 5 <noreply@anthropic.com>
Co-authored-by: dschaller <1004789+dschaller@users.noreply.github.com>
Co-authored-by: Derek <dschaller@users.noreply.github.com>

Author: 3 people