Change IPEX references / sections to be clearly informative (notional)

[pfeairheller]

During the 05/14/2024 community meeting we decided to move normative IPEX definitions to a new specification dedicated to IPEX.

For this specification, the section for IPEX should be reworded to make it clear that IPEX is not being normatively defined in ACDC spec but is informative on "how you could perform credential exchange".