Bring back RelationshipType (relationship name, from, to) to the model diagram

[bact]

Background

PR #935 improves diagrams by simplifying them to make their layout friendly for printed version (PDF, for example).
One of the strategies to simplifying the diagrams is to remove vocabulary entries.

Issue

While other vocabulary entries may seems redundant to be included in the model diagram, as one can directly look at the spec of each vocab easily (all entry names are at the beginning of each line), the RelationshipType is different as they involve with more information than just entry names and not all of them are in tabular-like format.

3 important information in RelationshipType are: 1) relationship name, 2) "from" Element, and 3) "to" Element.

While one can carefully read each line in https://spdx.github.io/spdx-spec/v3.0.1/model/Core/Vocabularies/RelationshipType/ for those information, the previously available diagram (shown below) clearly provides a quicker overview when one wants to check the "from" and "to", as they are in a tabular-like format. The table also group relationship types into sections with headings that based on their textual entry description, to assist the reader.

Proposal

• Bring back only the RelationType vocab, showing entry name, "from" Element, "to" Element in a tabular-like format (improve upon the previously available one)
• Adjust its layout to accommodate printed format

[goneall]

Changing to 3.1 milestone - @bact - let me know if you disagree

[bact]

No prob at all.