chore(docs): Add doc for task outputs permissions

[Milouu]

Add some docs on the default permissions settings for FedAvg and what the user can see in the front with/without permissions.

Closes FL-1174

[linear[bot]]

FL-1174 Explain task outputs permissions in the docs

# Context Permission task outputs is not a well understood concept so we need to document it. # Specification * Document it under the [Permissions paragraph](https://docs.substra.org/en/latest/documentation/concepts.html#permissions) in the Substra SDK docs * Explain how we use it (given an example for the FedAvg strategy) in the [Substra FL doc](https://docs.substra.org/en/latest/substrafl_doc/substrafl_overview.html). # Acceptance criteria Task outputs permissions concept is understandable by a new user.