Menu sidebar collapsed

## Summary

would it be possible to make the navbar directories collapse?
### Basic example

An effect similar to those of [docusaurus]( https://v2.docusaurus.io/docs/)
### Motivation

some topics are too long and going down the whole menu is heavy, in my opinion.

Start by locating the navbar and its directory-list rendering in the gridsome.org site, then compare the requested behavior with the linked Docusaurus example. Done means long topic directories can be collapsed and expanded so the menu is easier to navigate.

Written by the indexing model from the issue text.