Skip to main in tree view

[brichet]

Description

The skipToMain link has been added in #6844 to allow user to tab only once to reach the first Notebook cell.

But the link is also present in the tree view, and doesn't seem to do anything.

Should the link do something or should it be removed from the tree view ?

Reproduce

Open the tree view and tab (once there is no click anywhere else).

Expected behavior

The link to not exist or to redirect somewhere (maybe new notebook button, or the first notebook file in file browser...)