Flag dependency needs user documentation

## Acceptance criteria

- [ ] Flag Dependency is explained in a documentation page under "Managing Flags":
- It's explained what and why with end-user-targeted structure and text.
- Screenshots of the frontend are included using light theme.

Start by locating the documentation page or section under "Managing Flags" and review nearby pages for structure and style. Inspect the frontend Flag Dependency experience in the light theme to capture suitable screenshots. Done means the page explains what Flag Dependency is and why end users use it, with the required screenshots included.

Written by the indexing model from the issue text.