add Oxigraph in implementation page

[pchampin]

it was in the implementation report, but not on this page @Tpt

[afs]

suggestion to add the link to subj_RDF_ex_Elixir as well.

subj_Oxigraph_Rio_Turtle_Rust.

I already see RDF.ex in the implementation report.

[pchampin]

Thanks all for the comments. I updated the PR, following the example of the Ruby row. I also fixed a broken link for Ruby Turtle (now Ruby TriG).