Look into compatibility with current practices

As raised in https://github.com/solid/webid-profile/issues/68, and also discussed in https://github.com/solid/webid-profile/issues/65#issuecomment-1288210861, the WebID-Profile proposal i.m.o. conflates the normative and the descriptive, resulting in a number of problems with their document. The main defense of the proposal, voiced by @jeff-zucker, is that it wants to keep compatibility with the current way of doing things; something, he rightfully says, the SAI spec has up till now failed to do.

With this issue, I would therefore like to bring this lack of compatibility to he attention of the contributors, and ask that this is taken more into concern. On way to do so would be to split both SAI and WebID-Profile into separate mixed (sub)specs, and bundle our forces there, tackling one orthogonal problem after the other (as suggested in https://github.com/solid/data-interoperability-panel/issues/284).

Start by reading the compatibility concerns in webid-profile issues 68 and 65, then review the related proposal in data-interoperability-panel issue 284. Identify the current-practice gaps and decide whether splitting SAI and WebID-Profile into separate mixed subspecs addresses them; document the decision and resulting scope.