Use separate repo for api docs

[mmacata]

To be able to release the actinia API individually from the code, this PR starts to move openAPI descriptions to a separate repository https://github.com/mundialis/actinia-api.
This PR

• moves some of the API docs to actinia-api
• replaces their import
• adjusts the build to make use of the separate repository
• adjusts the build for updated plugins regarding api/v1 -> api/v2 changes
• add response to all old api/v1 endpoints. Example:

$ curl http://127.0.0.1:8088/api/v1/locations/nc_spm_08/mapsets | jq
[
  {
    "links": [
      "http://127.0.0.1:8088/api/v2/locations/nc_spm_08/mapsets"
    ],
    "message": "Are you looking for the current api version? Change 'v1' to 'v2' in the URL.",
    "status": "Not found"
  },
  404
]

Follow up:

• Move API doc descriptions completely to actinia-api #279

[griembauer]

Thank you, just one minor comment.