Add a security assurance case to the documentation (#5572)

Describe the threat model, the trust boundaries, the secure-design
argument, and how common weaknesses are countered, with links to the
quality assurance page as evidence.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

docs/mkdocs/mkdocs.yml

@@ -319,6 +319,7 @@ nav:
       - community/governance.md
       - community/roadmap.md
       - community/security_policy.md
+      - community/assurance_case.md
 
 # Extras
 extra: