Add localized mod documentation with syntax highlighting

manifest.json

@@ -8,6 +8,7 @@
   "sources": [
     "src/**/*.cs"
   ],
+  "test_sources": ["tests/*.cs"],
   "depends": ["ui_base>=1"],
   "main_menu": [
     {
@@ -17,6 +18,7 @@
     }
   ],
   "test_suites": [
-    { "id": "help", "activate": ["--itest=mod_help"] }
+    { "id": "help", "activate": ["--itest=mod_help"] },
+    { "id": "documentation", "activate": ["--itest=mod_documentation"] }
   ]
 }