Inconsistent handling of links in the releases resources

[odrotbohm]

The releases resource uses links inside the actual release representation, using …Url suffixes for the properties that contain URIs. It would be more idiomatic to rather move those to the _links section:

{
  "…",
  "_links" : {
    "api-doc" : {
      "href" : "…",
      "type" : "text/html"
    },
    "reference-docs" : [
      { "href" : "…", "type" : "text/html" },
      { "href" : "…", "type" : "application/pdf" }
    ]
  }
}

Note, how that would allow us to advertise different formats for the reference documentation (it's currently only one link, which is fine for now) and also indicate that the api-doc link points to a browsable resource.

This would be a non-breaking change, as the links can be added without disturbing existing clients. The reference docs could incentivize using the HAL links though.