Complete and automate publication of CSS RDF ontology

[tidoust]

#327 introduces a tool to create an RDF ontology for CSS properties, at the basis of https://www.w3.org/ns/css.ttl

The ontology only contains CSS properties from the CSS 2.2 specification and should be completed to include CSS properties defined in other specs. One difficulty there is that some CSS properties are defined more than once, see #127 (comment) so we'll need to apply some deduplication rule before merge.

Publication of the ontology should also be automated, typically done each time a new version of @webref/css gets published, otherwise the ontology will quickly become outdated.