docs(qa): add dedicated Matrix QA reference page

Adds a focused reference for the Docker-backed Matrix QA lane (CLI flags,
seven scenario profiles, eight env vars including the redaction toggle and
Tuwunel image override, scenario taxonomy, output artifact layout, and triage
tips). Source-of-truth checked against extensions/qa-matrix/src/cli.ts,
shared/live-transport-cli.ts, runners/contract/{runtime,scenario-catalog}.ts,
and substrate/harness.runtime.ts.

Registered in docs/docs.json alongside QA E2E automation.

docs/docs.json

@@ -1121,7 +1121,8 @@
                   "concepts/oauth",
                   "start/bootstrapping",
                   "concepts/experimental-features",
-                  "concepts/qa-e2e-automation"
+                  "concepts/qa-e2e-automation",
+                  "concepts/qa-matrix"
                 ]
               },
               {