Motivation

Resolve #2647

Have you read the Contributing Guidelines on pull requests?

Yes

Test Plan

Test on v2 website

1. In config file add a new new child item of nav that belongs to the same sidebar, for example:

    navbar: {
    // ...
      links: [
        {
          label: 'Docs',
          items: [
            {
              label: 'First item',
              to: 'docs/design-principles',
            },
            {
              label: 'Second item',
              to: 'docs/introduction',
            },

1. Run dev server

2. Click alternately on the first and second items - the sidebar should close.

Related PRs

(If this PR adds or changes functionality, please take some time to update the docs at https://github.com/facebook/docusaurus, and link to your PR here.)