Organize sections by conformance classes

Looking at the sequence diagram we can see 6 conformance classes, I'd like to clarify which section applies to which conformance class.

1. End User's WebID Document
   • WebID Issuer Discovery via Link Headers
2. OpenID Provider (OP)
   • Solid-OIDC Conformance Discovery
   • Requesting the WebID Claim using a Scope Value
   • DPoP Proof Validation
   • Client ID Document
   • Token Instantiation
   • DPoP-bound OIDC ID Token
   • Validate Client Identity (redirect_uri) Client ID Document
   • PKCE
3. Client's ID Document
   • Client ID Document
4. Client
   • Requesting the WebID Claim using a Scope Value
   • Authorization Server Discovery
   • Obtaining Access Token
   • PKCE
5. Resource Server (RS)
   • Authorization Server Discovery
6. Authorization Server (AS)
   • Obtaining Access Token
   • DPoP Proof Validation
   • OIDC ID Token Validation
   • WebID Issuer Discovery via Link Headers

I'll be updating the list above.

Besides tidying up the sections, we could add something similar to https://www.w3.org/TR/websub/#conformance-classes

Start from the Solid OIDC specification sections and the sequence diagram referenced in the issue. Map each section to the six listed conformance classes, then organize the specification accordingly and assess whether a WebSub-style conformance-classes section fits. Done means the section ownership is clear and the conformance classes are documented.