Improve documentation around backing types

[Weakky]

Description

The current docs about backing types are still using the nexus framework API https://nexusjs.org/docs/guides/schema#backing-types-in-nexus

We should convert them to be using the nexus schema API and also potentially make it a top-level guide or something as it's something core to Nexus that's fairly hidden.

Related #509

[Sytten]

I use a lot of them and I found a few tricks to work with prisma:

  typegenAutoConfig: {
    backingTypeMap: {
      DateTime: 'Date',
      Email: 'string',
      JSONObject: 'db.JsonObject',
    },
    contextType: 'ctx.Application',
    sources: [
      {
        alias: 'ctx',
        source: path.join(__dirname.replace(/\/dist$/, '/src'), './context/index.ts'),
      },
      {
        alias: 'db',
        source: '.prisma/client/index.d.ts',
      },
      {
        alias: 'models',
        source: path.join(__dirname.replace(/\/dist$/, '/src'), './models/index.ts'),
        typeMatch: type => [new RegExp(`(${type.name})`)], // Note: This will match any declared type in models if the name matches
      },
    ],
  },

So any type that I export in models automatically replaces the Prisma type. I also added some type backing in https://github.com/graphql-nexus/schema/pull/473