JSON-schema for create Command

[nifrhue]

Hello pdfcpu maintainers,

I've identified a potential enhancement that I believe could significantly benefit the wider pdfcpu community.

Feature Request:
I propose the introduction of a JSON Schema for the JSON input files used in the create command. This schema would define the structure, constraints, and types of data expected in the JSON files. For more information about JSON Schemas, please refer to json-schema.org.

Benefits:

1. Validation: A JSON Schema would enable developers to validate their JSON files against the expected format, reducing errors and streamlining the development process.
2. Clarity and Documentation: It would serve as both documentation and a guide for new users, clarifying the structure required for JSON input.
3. Development Efficiency: A clear schema simplifies the development and maintenance of scripts or applications that generate JSON inputs for pdfcpu.

Example Schema:
Below is a simple example of what a JSON Schema for a pdfcpu input might look like:

{
  "$schema": "https://json-schema.org/draft/2020-12/schema",
  "title": "PDFCPU Input",
  "type": "object",
  "properties": {
    "title": {
      "type": "string"
    },
    "author": {
      "type": "string"
    },
    "content": {
      "type": "array",
      "items": {
        "type": "object",
        "properties": {
          "text": { "type": "string" },
          "fontSize": { "type": "integer" },
          "color": { "type": "string" }
        },
        "required": ["text"]
      }
    }
  },
  "required": ["title", "content"]
}