[docs] Add multi-tenancy documentation

[RomuDeuxfois]

Context

Add documentation for the new multi-tenancy feature.

Also document the current limitation of the OpenCTI/OpenAEV interconnection:

• multiple OpenCTI instances connected to one OpenAEV instance: supported
• one OpenCTI instance connected to multiple OpenAEV instances: not supported

To support this second use case, multi-security-coverage support must first be implemented in OpenCTI.