Explicitly mention structure of a policy package in documentation

[rdimaio]

https://rucio.github.io/documentation/operator/policy_packages/#creating-a-policy-package

The docs mention the files needed in a policy package, but don't mention the actual structure required for the project to be properly built as a package.

By default, the structure of a package must be either src-based or flat: https://packaging.python.org/en/latest/tutorials/packaging-projects/