Include available translations in links

Automatically generate a links list of `available_translations` for a content item. 

The `available_translations` list is included in the related links hash using the same format as other related links.

This builds on #81 which should be merged before this.

- [x] update example output
- [x] update documentation
- [x] add a valiation to the links hash to prevent use of `available_translations` key
there's a slight nuance in that many of the tests build but do not persist the content item. 

The calculation of the `available_translations` list is done based on persisted data, so for a 'built' item, this list will be empty. 

Removing this conditional will have side effects on many existing tests and I took the view that it wasn't worth re-working all those tests to handle this edge case. 

It may be slightly confusing that a persisted item includes itself in the `available_translations` list but a non-persisted one does not, but I felt that in real-world usage we would only be dealing with persisted items, so again, I felt that it's not worth the extra work and code complexity to handle that particular scenario.