Add more detailed documentation on Artifacts

[mlodic]

As of now the Artifacts section in the doc is quite lean. See: https://intelowlproject.github.io/docs/IntelOwl/usage/#analyzables-artifacts

We should add documentation about:

• how to add a new artifact: explain where it is possible to do that, how basic/advanced evaluations work, how decay works, and so on.

It is important to do and share in the doc all possible screenshots from the app that can help the understanding.