fix: add pentest architecture to sidebar, fix image paths, enable mermaid

- Add vitepress-plugin-mermaid for diagram rendering
- Add Pentest Architecture page to sidebar nav
- Fix image paths (../public/ → / for VitePress public serving)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docs/package.json

@@ -8,5 +8,9 @@
   },
   "devDependencies": {
     "vitepress": "^1.6.4"
+  },
+  "dependencies": {
+    "mermaid": "^11.13.0",
+    "vitepress-plugin-mermaid": "^2.0.17"
   }
 }