feat(devcontainer): add forwardPorts 3000

Required for testing the documentation with the host's browser.

PR: #477

.devcontainer/devcontainer.json

@@ -7,5 +7,6 @@
   "extensions": ["ms-vscode.cpptools"],
   "settings": {
     "terminal.integrated.shell.linux": "/bin/bash"
-  }
+  },
+  "forwardPorts": [3000]
 }