documentation: Align with latest changes

Update the overview to reference external definitions of the Primary
Compute and Safety Island.

Align with current state of the repository.

Modify changelog to reflect the expected upcoming v1.0 release.

Issue-Id: SCM-5892
Signed-off-by: Ambroise Vincent <ambroise.vincent@arm.com>
Change-Id: I0f1d080cc2f23c4561f9b78ac7bd12b795fde19c