Review of RDF Concepts

Dear all,

Here is my review of RDF Concepts (as a diff against my fork):
https://github.com/w3c/rdf-concepts/compare/main...niklasl:rdf-concepts:rdf12proposition?expand=1

A preview of the results of those changes:
https://raw.githack.com/niklasl/rdf-concepts/rdf12proposition/spec/index.html

In summary, this suggests to:
* Correct an initial missing mention of triple terms as an element of
triples.
* Add some text about propositions, attempting to clarify the concept that
triple terms denote, and therefore their intended usage.
* Use a variant of the triple term illustrations, based on suggestions by
Pierre-Antoine for the corresponding illustration in the Primer PR.
* Clarify the difference between a triple and a triple term.

Others have made other suggestions in their reviews, many of which I also
agree with. I have not incorporated those into this.

Best regards,
Niklas