feat: add docs links to error responses

[MartinKolarik]

• Deletes the old docs.
• Adds links to the new docs into all error responses (logic reused from the jsDelivr API):

{
  "error": {
    "type": "validation_error",
    "message": "Parameter validation failed.",
    "params": {
      "limit": "\"limit\" must be less than or equal to 500"
    }
  },
  "links": {
    "documentation": "https://www.jsdelivr.com/docs/api.globalping.io#post-/v1/measurements"
  }
}