Add an actors' model

[thomas-fossati]

Add a section that describes the expected PSA supply-chain actors:

• ODM
• (maybe) ISV
• certification body

Say what kind of material they use to assemble their CoRIMs.

Note As per discussion in Veraison public meeting (27/04/2021):

• keep supply chain actors open, i.e. not rigidly coupled as this is very much dependent on the deployment / use case.