feat(sidebars): add support for depth and nested

[fiji-flo]

Description

Add support for:

sidebar:
  - type: section
    link: /Glossary
    title: Glossary
  - type: listSubPages
    path: /Glossary
    depth: 2

and

sidebar:
  - type: section
    link: /Glossary
    title: Glossary
  - type: listSubPages
    path: /Glossary
    depth: 2
    nested: true

This doesn't affect the current sidebars until the new properties are added.

Motivation

We're missing css function in the csssidebar and we cloud enhance the glossary sidebar, too.

Related issues and pull requests

Fixes #73