[doc/security] Update security landing page.
[doc/security] Update security landing page.

*   Update the page title to OpenTitan Security.
*   Add a new section pointing to the security model architecture docs.
*   Update the list of security primitives with references to specific
    comportable IP specifications.
*   Updated overall document references.

Signed-off-by: Miguel Osorio <c7c97b215f812c2e8a32efd57ad5e976a6eb9e45@google.com>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan