[backend] Add documentation pipeline execution schedule policy schema

We plan the introduction of a new policy type and schema in #504088 (closed). We will add the feature behind a feature flag. Documenting the schema right away could be misleading until the feature actually works. We should add the documentation as soon as we have a working MVC.

We expect workflow rules in the new feature to work differently from pipeline execution policies. We should also document the differences.

Schema proposal

"pipeline_execution_schedule_policy": {
  "type": "array",
  "description": "Starts pipelines on a schedule with custom pipeline configuration.",
  "additionalItems": false,
  "maxItems": 5,
  "items": {
    "required": [
      "name",
      "enabled",
      "content",
      "schedule"
    ],
    "type": "object",
    "properties": {
      "name": {
        "description": "Name for the policy.",
        "minLength": 1,
        "maxLength": 255,
        "type": "string"
      },
      "description": {
        "description": "Specifies the longer description of the policy.",
        "type": "string"
      },
      "content": {
        "description": "Specifies the content of custom configuration.",
        "type": "object",
        "properties": {
          "include": {
            "type": "array",
            "maxItems": 1,
            "minItems": 1,
            "items": {
              "type": "object",
              "properties": {
                "project": {
                  "type": "string"
                },
                "file": {
                  "type": "string"
                },
                "ref": {
                  "type": "string"
                }
              },
              "required": [
                "project",
                "file"
              ],
              "additionalProperties": false
            }
          }
        },
        "required": [
          "include"
        ],
        "additionalProperties": false
      },
      "schedule": {
        "description": "Specifies the condition when this policy should be triggered.",
        "type": "object",
        "properties": {
          "cadence": {
            "description": "Specifies when this policy should schedule a new pipeline with enforced `actions`. Uses cron expression as a format (ie. `0 22 * * 1-5`). Supported only when `type` is set to `schedule`.",
            "type": "string",
            "pattern": "(@(yearly|annually|monthly|weekly|daily|midnight|noon|hourly))|(((\\*|(\\-?\\d+\\,?)+)(\\/\\d+)?|last|L|(sun|mon|tue|wed|thu|fri|sat|SUN|MON|TUE|WED|THU|FRI|SAT\\-|\\,)+|(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec|JAN|FEB|MAR|APR|MAY|JUN|JUL|AUG|SEP|OCT|NOV|DEC|\\-|\\,)+)\\s?){5,6}"
          },
          "timezone": {
            "type": "string",
            "description": "Time zone to apply to the cadence. Value must be an IANA Time Zone Database identifier, for example: `America/New_York`."
          }
        },
        "required": [
          "cadence"
        ],
        "additionalProperties": false
      },
      "policy_scope": {
        "$ref": "#/$defs/policy_scope"
      },
      "metadata": {
        "$ref": "#/$defs/metadata"
      },
      "enabled": {
        "description": "Whether to enforce this policy or not.",
        "type": "boolean"
      }
    }
  }
}