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
This is only true in the storing context.  Might be good to make that clear.