docs: static documentation site generator and authored pages

Twenty-six pages covering concepts, architecture, every subsystem, an
interface tour with screenshots, the full API route table generated from
openapi.json, the CLI, security and threat model, operations, ADRs,
development history and a glossary. Built into the dixon.cx page skeleton,
unlinked and noindex.

docs/site/pages/security.md

@@ -0,0 +1,7 @@
+Noema treats every external byte as hostile, every action as something that must pass policy, and every secret as something that must never come back out. This page reproduces the security model, the authentication design and the threat model with the test that verifies each mitigation.
+
+{{include SECURITY.md}}
+
+## Threat model
+
+{{include THREAT_MODEL.md demote=1}}