Add source_for relation type for forward provenance

[emmanuelmathot]

Introduce the source_for relation type to support forward provenance, allowing a source item to link to derived items. This addition enhances the documentation and changelog to reflect the new relation type, which is the inverse of derived_from.

Fixes #27

[m-mohr]

Generally a good idea, I'm just trying to remember whether I have seen this before (which I think) and whether it was using a different relation type. Trying to find it again, maybe worth checking with OGC (Provenance?) as well.

(Side note: It's a bit of a pity that we use _ here and in derived_from while in (all?) other relations we use -)