Fix links by reviewing the whole spec

[chrdebru]

For instance, in Section 2.2 Mapping Graphs and the RML Vocabulary, there are a bunch of links to the spec and not a specific source. Specific nodes cannot be found in the spec either (e.g., Logical Source).

[DylanVanAssche]

@dachafra I can have a look if things are published, it is hard to keep track of all the branches going around.

[dachafra]

ok, let's then add the label pending here until the next version will be ready

[pmaria]

I fixed a bunch in my last pull request, but there are still many links that should be definition references. Still WIP

[dachafra]

This will be done after all updates and contributions are merged into the spec