Fix description of Application Roles article

**Product:** Tarantool
**Root document:** https://www.tarantool.io/en/doc/latest/platform/app/app_roles/#creating-a-custom-role
**SME:** @ totktonada

# Details

* Move description of the ``on_event`` callback to a separate subsection;
* Add link from role creation steps to all subsections mentioned in the steps (configuration schema, validation function, apply function, etc.)