Add documentation for CWE 319.1

What does this MR do and why?

Add documentation for CWE 319.1 As the part of https://gitlab.com/gitlab-org/gitlab/-/issues/331216 we are adding the CWE 319.1 i.e Mixed Content warning to the browserker(dast) check.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #331216