bug: content location is not configurable

[airtonix]

I see that you've hardcoded where the content documents are assumed to be:

https://github.com/JulianCataldo/astro-content/blob/master/packages/server/setup.ts#L60-L62

it'd be amazing if that could part of the integration Settings type:

https://github.com/JulianCataldo/astro-content/blob/master/packages/types/user-config.ts#L3-L10

export interface UserConfig {
  /** **Default**: `/` */
  previewUrl?: string;
  /** **Default**: `'info'` */
  logLevel?: LogLevel;
  /** **Default**: `true` */
  gui?: boolean;
  /** **Default**: `/content` */
  contentSource?: string;
}