doc: change to move security.md

The security.md file is currently within the doc folder,
however this is not being located by Google Scorecard.

To address this issue, it is being moved to the main directory
of the project.

Signed-off-by: Katherine Vincent <katherine.vincent@arm.com>
Change-Id: Ibb9dba4fa284215111ab458df4e5a86815c09c6e