This post has a custom navigation list set in the post’s YAML Front Matter.

sidebar:
  title: "Sample Title"
  nav: sidebar-sample

Along with navigation elements set in _data/navigation.yml.

sidebar-sample:
  - title: "Parent Page A"
    children:
      - title: "Child Page A1"
        url: /
      - title: "Child Page A2"
        url: /
      - title: "Child Page A3"
        url: /
      - title: "Child Page A4"
        url: /
  - title: "Parent Page B"
    children:
      - title: "Child Page B1"
        url: /
      - title: "Child Page B2"
        url: /
      - title: "Child Page B3"
        url: /
      - title: "Child Page B4"
        url: /
      - title: "Child Page B5"
        url: /
  - title: "Parent Page C"
    children:
      - title: "Child Page C1"
        url: /
      - title: "Child Page C2"
        url: /
      - title: "Child Page C3"
        url: /
      - title: "Child Page C4"
        url: /
      - title: "Child Page C5"
        url: /
  - title: "Parent Page D"
    children:
      - title: "Child Page D1"
        url: /
      - title: "Child Page D2"
        url: /