Add support for documentation generation

[joshmossas]

We should be able to support generating a complete API documentation site based on definition.json file similar to open API. It could work something like this.

arri document --definition [path-to-defitiion.json] --output [output-directory]

Ideally it would also detect what code generators are being used in the arri.config.ts and produce sample code for each client language being generated.

There also the possibility of just integrating this as a clientGenerator plugin:

import { defineConfig } from 'arri';
import { documentationGenerator } from 'arri/dist/codegen';

export default defineConfig({
  clientGenerators: [
    documentationGenerator()
  ]
});