Refactor model links, describe link in description

- Add description of linked/embedded resources to service description
- Pass command to ResourceBuilder instead of client
- Remove "deskClass" data from service description
- Remove InvalidEmbedFormatException (not needed any more)
- Remove custom concrete command class Desk\Relationship\Command (not
  needed any more)
Improve internal documentation

- Add various type hints where possible
- Removed unused "use" statements
- Fix some inaccurate PHPDoc blocks