ldp:contains doesn't necessarly point to the focus node of Data Instance

In 5.2. Data Registration we find

Each Data Instance is linked via the ldp:contains property.

In cases where IRI denoting Data Instance has # fragment, ldp:contains will just link to the resource without the fragment.

I understand that we want to rely on the server to manage containment tripes. In that case we probably need to clarify that ldp:cotains links to the resource describing data instance and explain how do find focus node of that resource.

I'm going to update snippets in the primer and instead of having

alice-pro:773605f0-b5bf-4d46-878d-5c167eac8b5d
  a interop:DataRegistration ;
  # [...]
  interop:registeredShapeTree solidtrees:Project ;
  ldp:contains
    alice-pro:ccbd77ae-f769-4e07-b41f-5136501e13e7\#project ,
    alice-pro:7a130c38-668a-4775-821a-08b38f2306fb\#project .

have it without fragments

alice-pro:773605f0-b5bf-4d46-878d-5c167eac8b5d
  a interop:DataRegistration ;
  # [...]
  interop:registeredShapeTree solidtrees:Project ;
  ldp:contains
    alice-pro:ccbd77ae-f769-4e07-b41f-5136501e13e7 ,
    alice-pro:7a130c38-668a-4775-821a-08b38f2306fb .

Once I do that I will see where sai-js code breaks and where do we need to map resources to their focus nodes.

Start with section 5.2, Data Registration, and the primer snippets described in the issue. Review how ldp:contains is explained when a Data Instance IRI has a fragment, update the examples to use resource IRIs without fragments, and clarify how to find the resource's focus node; then check where the sai-js code may depend on the old form.