[Enhancement] Extend Gridsome.org Appearance

#### 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
https://github.com/vuejs/vuepress/issues/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._**