Nested sidebar groups support

[youseiraws]

Feature request

What problem does this feature solve?

sidebar: {
            '/foo/': [{
                title: 'parent',
                children: [{
                    title: 'child',
                    children: [
                        'bar/01',
                        'bar/02',
                        'bar/03'
                    ]
                }]
            }]
        },

Nested sidebar groups can not support.I want get this feature.Thanks!

What does the proposed API look like?

How should this be implemented in your opinion?

Are you willing to work on this yourself?**

[ulivz]

Duplicate of #287 and closing it in favor of this PR: #669