Add link to documentation site from JSON Schema

For each configuration option, we should add a link to the docs site, where there is usually more documentation for that given config option

i.e.

    "abandonmentThreshold": {
      "description": "Flags packages that have not been updated within this period as abandoned.\nSee also: https://docs.renovatebot.com/configuration-options/#abandonmentthreshold",
      "type": [
        "string",
        "null"
      ],
      "default": null
    },

We'll need to make sure to route Repo or Self-Hosted config option to the correct page.

(There may be other fields we can use to surface this)