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

---

The other related issue #254

[netlify[bot]]

✅ Deploy Preview for hugo-hextra ready!

Name	Link
🔨 Latest commit	9e7b13a0dae76dab674c7ee76f5d1977d77f00a3
🔍 Latest deploy log	https://app.netlify.com/sites/hugo-hextra/deploys/664687e2af92290008b47af8
😎 Deploy Preview	https://deploy-preview-376--hugo-hextra.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify site configuration.