Global nav: 2nd+ iteration

• Follow up from https://gitlab.com/gitlab-com/gitlab-docs/merge_requests/362 (MVC)

What

For all the planned improvements (and context) see gitlab-com&21 (closed).

• Fix ToC bug: https://gitlab.com/gitlab-com/gitlab-docs/issues/299 // https://gitlab.com/gitlab-com/gitlab-docs/merge_requests/374
• Fix known issues: gitlab-com&21 (closed)
• Add nav for Omnibus and Runner: #409 (closed)
• Add capability to expand and collapse second-level menus, hiding third-level items by default
• Add button to toggle the left sidebar (something that works well on mobile too) => https://gitlab.com/gitlab-com/gitlab-docs/issues/336
• Address feedback from users and team
  • Link API docs => https://gitlab.com/gitlab-com/gitlab-docs/merge_requests/375
  • Improve contrast => https://gitlab.com/gitlab-com/gitlab-docs/merge_requests/376
  • Fix JS hiding the last links on Firefox => https://gitlab.com/gitlab-com/gitlab-docs/issues/303

cc/ @gl-docsteam