body {
  margin: 0;
  font-family: sans-serif;
  display: flex;
}
nav {
  width: 220px;
  background: #f8f8f8;
  padding: 1rem;
  height: 100vh;
  overflow-y: auto;
  position: sticky;
  top: 0;
}
main {
  padding: 2rem;
  flex: 1;
}
nav a {
  display: block;
  color: #333;
  text-decoration: none;
  margin: 0.5rem 0;
}
nav .section-title {
  font-weight: bold;
  margin-top: 1rem;
}