Rules draft

[afs]

Working copy PR for completing the rules document.
Mostly small changes in sections across the document.

This makes changes visible, while batching them into one for a final approval. This recognizing that at this time of year, there is less approvals capacity. It is somtimes tricky to continue editing when there are changes in pending PRs.

See the live editors draft online here

Tasks to progress:

• Create a running example for the rules description section #930
• SHACL Rule grammar - DATA should not allow variables. #734
• Abbreviations : TRANSITIVE, SYMMETRIC, and INVERSE #779
• RDF syntax for SHACL Rules #870 - more necessary/needed?

This closes #769
This closes #956
This closes #516

[afs]

CSS styling of examples restructured.
CSS simplified and reduced.
It should be practical to tweak example styling and have that consistently appear.

Dark mode should work.

Caveat when switch in/from dark mode - their is general issue with W3C docs that the light/dark setting can be quite sticky and if you change between documents it gets confused. A hard refresh might be needed.

[afs]

Merged - and a new PR started - so that the changes so far are visible on GH pages and /TR/.