Adjust the primer towards OIDC Federation

[elf-pavlik]

Part of #207 This draft is intended as collaborative scratchpad. We can use inline suggestions to propose and discuss changes.

TODO

• [ ] OP advertises automatic registration
• [ ] authentication request should use Request Object or PAR
• [ ] RP should publish the trust chain
• [ ] OP should validate the trust chain
• [ ] in the abstract en intro, refer to OpenID Federation, next to OpenID Connect;
• [ ] switch "Client ID Document" to "Entity Statement" or "Entity Configuration".

[elf-pavlik]

@acoburn I think we should have a simplest case with all the details. Since this is a primer I don't see a problem with showcasing something which isn't specified directly in Solid-OIDC but in OIDC Federation or DPoP.