refactor: render sidebar from data

[imfing]

this is a major refactor aiming to address the sidebar performance issue described in #122

• enable render the sidebar from a nested data, e.g. data/en/sidebar.yaml:

docs:
  - title: Documentation
    link: /docs/
  - title: Guide
    link: /docs/guide/
    open: false
    items:
      - title: Organize Files
        link: /docs/guide/organize-files/

• add optional flags to enable caching the sidebar (experimental)

params:
  page:
    sidebar:
      enable: true
      source: data  # auto | data
      cache: true

• when sidebar is cached, use JavaScript to expand sidebar automatically to the current page

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The other related issue #254

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