SHACL shapes for validating TREE servers

[pietercolpaert]

It would be great for server implementers to also have a way to validate the pages they create. Therefore it would be nice that they would have standardized SHACL shapes for e.g., validating:

1. The root node -- as this node has extra requirements on the descriptions
2. A subsequent node -- as this does not have a requirement to repeat the descriptions on the root node
3. A page used for discovery -- this is still in the works by @julianrojas87 as a dedicated work item in the TREE CG Meetings

@rorlic already did an amazing feat of strength and created an overview of SHACL test cases for producers in https://github.com/rorlic/tree-shacl -- I believe these should be discussed in our CG and adopted.

[xdxxxdx]

My concerns here: rorlic/tree-shacl#1.