YAML configuration documentation

[bushsf]

Please update the documentation so that the QKD examples will work by updating the link_layer option to false:

Adding a YAML comment about the link_layer option would be helpful.
More YAML comments would be helpful, e.g.:

1. qkd_source_id is just a reference and does not set anything
2. what are all min and max values, where applicable
3. what does "levels" mean?
4. what are the implications of turning on the link_layer as described above.
5. what "units" are supported?
6. What is "subscriber_id"?
7. Where is the key cache in max_key_cache_size? What happens if cache overflows?
8. An explanation of the relation between the listen and connect port addresses in initiator and responder would be helpful.