[docs] Add Sphinx-based documentation (not built by default)

Initial work on documenting the module and also added some comments (in the documentation) how we should probably sanitize the API as starting point for discussion.
Yeah... I don't know why I called it destination although the discrepancy between types can somewhat be explained by DBusAdaptor coming later and destination making no sense as a property of the recipient.

Deprecation of destination in favour of service is probably a good idea.