update security ima

The IMA paragraph in the Apertis security section contains some
non-valuable information as well as an external link that cannot
be resolved because the referenced page was shut down.
Both parts were removed.

Signed-off-by: Tino Lippold <Tino.Lippold@de.bosch.com>

Correct external links in designs/security section

See merge request Tino.Lippold-ext/apertis-website!3

Correct several external links in Apertis designs/security section.

Signed-off-by: Tino Lippold <Tino.Lippold@de.bosch.com>

Update security.md

See merge request Tino.Lippold-ext/apertis-website!1