docs(threat-model): refresh top-level page

docs(threat-model): refresh top-level page

The top-level page for threat model documents is evidently out-dated,
as it contains text which no longer makes sense on its own. Most
likely it relates back to the days where we had a single threat model
document.

Reword it accordingly. While we are at it, explain the motivation and
structure of the documents.

Change-Id: I63c8f38ec32b6edbfd1b4332eeaca19a01ae70e9
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>