Add illustrations for policy cards

Purpose

Add small illustrations to be used for policy cards (3) (see #327 (closed) for more). Illustration names and concepts are designed for reuse and a more universal approach to help reduce the number of 1-up illustrations in the library. For example, the small illustration for Merge Request Approval Policy can also be used for other "secure" concepts in the UI.

Screenshots

MR Security Approval (secure-sm)	Scan Execution (search-sm)	Vulnerability Management (scan-alert-sm)

Checklist

For a new icon and/or illustration:

• Link to related design issue:
• If this adds an icon ensure that it follows the new icon checklists.
• If this adds an illustration ensure that it follows the new illustration checklists.

For third-party trademarks:

• If this adds or updates an icon or illustration that includes a third-party trademark, ensure that it follows the process for adding third-party trademarks to GitLab.

Closes #327 (closed)