prevent sidebar from re-rendering

## Feature request

Each time we navigate, if the sidebar menu item expands, the whole sidebar re-renders (sidebar DOM is replaced with a new one)

#### What problem does this feature solve?

This would make it easier to attach logic onto DOM elements without the being destroyed.

#### What does the proposed API look like?

We should render all menu items with all their child items, but all collapsed. We can then open/close them to reveal the children. This is better than re-rendering the whole sidebar in order to add/remove the child content.

#### How should this be implemented in your opinion?

We can use a pure CSS tree view (similar to the ones I linked in https://github.com/docsifyjs/docsify/issues/1180)

#### Are you willing to work on this yourself?

Yeah