Add security documentation

### Enhancement Proposal

We should write a new Explanation page around security. The page should address any security-related design decisions we made with this charm, and the common risks and best practices for using the charm. The page should address all possible vulnerabilities and risks, including:

* The charm allows the URL to be supplied to an external script which might be source of vulnerability.

Upon completion, the Explanation page will provide clarity and reassurance to users that this charm will provide a secure and reliable image builder solution for their use cases.

### Impact

Medium (The feature may be helpful in the future)

### Impact Rationale

_No response_