added RDFa annotations to index.html (this has no adverse effect on the layout!)

FYI:
* This was suggested by an author.
* It makes particular sense for a workshop on semantic publishing.
* It also makes sense in general to help search engines
* Direct embedding of RDF into index.html is much faster than waiting for DBLP to index it and a third party to update the linked open dataset derived from DBLP.
* I'm going to add this feature to https://github.com/clange/ceur-make
