Port the Ontop native mapping syntax to the website

[bcogrel]

Currently at https://github.com/ontop/ontop/wiki/TurtleSyntax and https://github.com/ontop/ontop/wiki/URILiteralTemplate#URI_Template

• [ ] Move content
• [ ] Update language tags (have to be constant now)
• [ ] Add named graph
• [ ] Do not list supported data types there (not relevant anymore)
• [ ] Make sure we don't suggest people using rdfs:literal as datatype (meaningless as there is no ambiguity whether something is a literal or an IRI)
• [ ] Check for other changes

[bcogrel]

Also https://github.com/ontop/ontop/wiki/ontopOBDAModel and https://github.com/ontop/ontop/wiki/ObdaFileFormat

[bcogrel]

Suggestion from @namedgraph: adding the named graph syntax to the tutorial as well