📝(docs) light doc about org & SP

This introduces a very light documentation about
the newly introduced models.

docs/organizations.md

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+# Organization model
+
+## Purpose
+
+The initial `Organization` model allows to regroup `User` and `Team` under a same object.
+
+While the purpose was purely technical in the first place, few features emerged afterward.
+
+
+## Link with the `User` model
+
+Each user must have a "default" organization. 
+
+When a user logs in:
+
+- The backend tries to get an existing organization from a "registration ID" or from the user's mail domain
+- If the organization does not exist, it is created using the "registration ID" and fallbacks on the user's mail domain
+- The user organization is set to this organization.
+
+**Note:** While deploying the new code, we allow the already existing user to not have one.
+
+
+## Link with the `Team` model
+
+Each team must have an organization. This organization is not defined by the user, but automatically set
+using the user's organization.
+
+The team's organization does not restrict the users who can be added to the team.
+
+There is currently no feature relying on the team organization.
+
+**Note:** While deploying the new code, we allow the teams to not have one.
+
+
+## Organization-related features
+
+### Organization UI
+
+A new interface can be created to allow users to see all members of an organization. 
+This could be used along with the contacts.
+