Process Examples incomplete

[m-mohr]

Process examples can consist of either arguments only or a process graph at the moment. For process graphs, I probably have forgotten to update the process examples structure when doing the changes for #161. Looking through the processes, the process_graph variant of the examples is rarely used yet (see also Open-EO/openeo-processes#136 ). Another issue is that adding multiple "process graph" examples to a process will increase the size of the GET /results response by far. Also many examples get duplicated. Thus I propose the following:

• Remove process graph examples from the processes + API
• Add A rel type "example" to the list and allow linking to examples externally. This decreases the response size and makes re-using examples easier.

Strictly speaking, this is a breaking change, but I think it's very rarely used at the moment (maybe even not used at all) and it makes the spec cleaner.