Capability to navigate docs using tabs

[catherineluse]

The search button (magnifying glass) and the drop-down menus in the left navigation bar of the documentation need to be accessible by tabbing through the interface.

The rancherlabs/website repo was the wrong repository, so I'm moving this issue here. https://github.com/rancherlabs/website/issues/1942 https://github.com/rancher/docs/issues/2306