DPPT-970: Codify scan type beams

Codify the existing keys in the "beams" entry in the scan types and add the "integration_time" key. The "scan_types" part of the execution block looks like this (ignoring other keys):

{
  "execution_block": {
    "scan_types": [
      {
        "scan_type_id": "...",
        "beams": {
          "vis0": {
            "field_id": "...",
            "channels_id": "...",
            "polarisations_id": "...",
            "integration_time": 0.125
          }
        }
      }
    ]
  }
}

I have taken the opportunity to remedy an omission from the documentation. The "max_length" key in the execution block, which sets its maximum length in seconds, was described in AssignResources 0.3. It unfortunately lost its description in version 0.4 onwards, possibly because the schema was rearranged. I have added the description for the more recent versions.