docs: fix user mode sidebar

The sidebar for User Mode wasn't set up correctly and
it wasn't obvious that there are linked sub-pages with
design details about memory domains, system calls, kernel
objects, and so forth.

Split the introductory material into its own overview
page and set up the table of contents properly.

The text of the threat model, high level policy details,
and constraints sections is unchanged.

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>