Initial multitenancy implementation

docs/data-model/tenancy.md

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+NetBox supports the concept of individual tenants within its parent organization. Typically, these are used to represent individual customers or internal departments.
+
+# Tenants
+
+A tenant represents a discrete organization. Certain resources within NetBox can be assigned to a tenant. This makes it very convenient to track which resources are assigned to which customers, for instance.
+
+### Tenant Groups
+
+Tenants are grouped by type. For instance, you might create one group called "Customers" and one called "Acquisitions."