Add overlay for simplified Duration data type

[lcawl]

This PR adds an overlay to change the definition for the Duration data type in the OpenAPI files that are used for docs.

It was:

      "_types.Duration": {
        "description": "A duration. Units can be `nanos`, `micros`, `ms` (milliseconds), `s` (seconds), `m` (minutes), `h` (hours) and\n`d` (days). Also accepts \"0\" without a unit and \"-1\" to indicate an unspecified value.",
        "oneOf": [
          {
            "type": "string"
          },
          {
            "type": "string",
            "enum": [
              "-1"
            ]
          },
          {
            "type": "string",
            "enum": [
              "0"
            ]
          }
        ]
      },

This PR changes it to:

      "_types.Duration": {
        "description": "A duration. Units can be `nanos`, `micros`, `ms` (milliseconds), `s` (seconds), `m` (minutes), `h` (hours) and\n`d` (days). Also accepts \"0\" without a unit and \"-1\" to indicate an unspecified value.",
        "type": "string"
      },

The reason for the change is that the way it's currently rendered in the documentation, it looks like "-1" and "0" are the only valid options:

For example in https://www.elastic.co/docs/api/doc/elasticsearch/operation/operation-snapshot-repository-analyze

With this overlay in place, the output is as follows:

[github-actions]

Following you can find the validation changes against the target branch for the APIs.

No changes detected.

You can validate these APIs yourself by using the make validate target.

[leemthompo]

Preview looks good to me, but I guess it would be nice to indicate potential valid values, but sounds like a can of worms per the Slack discussion. It's at least nice to remove the implication that it only accepts -1 or 0.