Keyboard shortcuts in the TabFern window

@r4j4h Following on from #8, which was merged in the course of the push to v0.1.
Besides the TODO item there, the other thing I would appreciate along with the code is a bit more documentation. At this point, the options and the default bindings don't look to me like they would tell a new user what the shortcuts were for. Thanks!

Start by reading issue #8 and inspecting the existing options and default keyboard bindings in the TabFern window. Document what the shortcuts do so a new user can understand their purpose; the work is done when the available bindings and their uses are clear.

Written by the indexing model from the issue text.