[Enhancement] Extend Gridsome.org Appearance

[R9980]

This's a suggestion for enhance gridsome.org sidebar. It'll definitely helpful for others who want to be Gridsome Users

Sidebar

Current
No menu levels. just single topic list under each category.

Expect
Multiple menu levels(groups)/nested children
https://vuepress.vuejs.org/default-theme-config/#sidebar
vuejs/vuepress#1497
https://github.com/timaschew/vuepress-nested-sidebar-demo (source)
https://hungry-yonath-bfd0a1.netlify.com

(Keep exist gridsome sidebar ui and effects. just like it and better than vuepress)

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If no consideration for extend default gridsome documentation as mentioned above, atleast prefer request for add proper guide, tutorial for necessary users to follow.

[R9980]

https://docdock.netlify.com (That's better than above resources)
https://github.com/vjeantet/hugo-theme-docdock

[samuelhorn]

I think the current structure works good as is. But maybe we could scan trough the MD files, adding h2s as new foldable sub anchors under existing first level links. By doing that, templates (https://gridsome.org/docs/templates/) for example becomes:

• Templates ▼ (link to page with folding arrow)
  -- Setup templates (anchor on page)
  -- Add data to a template (anchor on page)
  -- Node fields as meta info (anchor on page)

I don't think deeper level than that is needed?

If that sound good I could fix it.

[TerabyteTiger]

Is this similar to #514 (and associated PR #532)?