docs: add security model documentation

[alexandre-daubois]

The ecosystem security team of the foundation needs additional information on the trust model of FrankenPHP to ensure valid security reports.

This document adds information about what's considered a security issue or not in FrankenPHP, before we can run new scans on the project.

[Copilot]

Pull request overview

Adds security model documentation that defines FrankenPHP’s trust boundaries and clarifies which vulnerability classes belong to FrankenPHP versus applications or upstream dependencies.

Changes:

• Adds docs/security.md covering trust boundaries, data taint, in-scope attack surfaces, and out-of-scope issues.
• Links the security model from the root SECURITY.md.
• Adds the new security page to llms.txt.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
SECURITY.md	Adds a Security Model section linking to the new documentation.
llms.txt	Adds the security model page to the documentation index.
docs/security.md	Introduces the new security model and scope guidance.

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