The policy at https://flarum.org/privacy is specifically for this forum and the other Flarum-owned websites. It can serve as an example, but what you need will depend on how you configured Flarum and which extensions you used.
A standard Flarum installation collects very little information about the users. It's basically what is asked during signup, plus the IP address is logged every time a user posts. Flarum only uses cookies for the purpose of providing login.
Extensions that add analytics or advertisement will obviously collect additional data. Often they use well-known services like Google Analytics so you should be able to find information about what those third-party collect in their own documentation.
