Add an example of a relationship union list type

examples/usecase-relationship-union/README.md

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+## Base Project - Versioning
+
+This project demonstrates a Posts list where a particular field is always required to be sent in a GraphQL request; this can useful for optimistic locking and versioning.
+
+Use it as a starting place for learning how to use Keystone.
+
+## Instructions
+
+To run this project, clone the Keystone repository locally, run `pnpm` at the root of this repository, then navigate to this directory and run:
+
+```shell
+pnpm dev
+```
+
+This will start the Admin UI at [localhost:3000](http://localhost:3000).
+You can use the Admin UI to create items in your database.
+
+You can also access a GraphQL Playground at [localhost:3000/api/graphql](http://localhost:3000/api/graphql), which allows you to directly run GraphQL queries and mutations.
+
+Congratulations, you're now up and running with Keystone! 🚀
+
+### Optional: add sample data
+
+This example includes sample data. To add it to your database:
+
+1. Ensure you’ve initialised your project with `pnpm dev` at least once.
+2. Run `pnpm seed-data`. This will populate your database with sample content.
+3. Run `pnpm dev` again to startup Admin UI with sample data in place.
+
+## Try it out in CodeSandbox 🧪
+
+You can play with this example online in a web browser using the free [codesandbox.io](https://codesandbox.io/) service. To launch this example, open the URL <https://githubbox.com/keystonejs/keystone/tree/main/examples/usecase-versioning>. You can also fork this sandbox to make your own changes.
+
+## Next steps
+
+Experiment with the code in this example to see how Keystone works, familiarise yourself with the Admin UI, and learn about the GraphQL Playground.
+
+When you’ve got the hang of this base project, try a [feature project](../) to learn Keystone’s advanced features and take your knowledge to the next level.

examples/usecase-relationship-union/keystone.ts

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+import { config } from '@keystone-6/core';
+import { fixPrismaPath } from '../example-utils';
+import { lists } from './schema';
+import { TypeInfo } from '.keystone/types';
+
+export default config<TypeInfo>({
+  db: {
+    provider: 'sqlite',
+    url: process.env.DATABASE_URL || 'file:./keystone-example.db',
+
+    // WARNING: this is only needed for our monorepo examples, dont do this
+    ...fixPrismaPath,
+  },
+  lists,
+});

examples/usecase-relationship-union/package.json

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+{
+  "name": "@keystone-6/example-usecase-relationship-union",
+  "version": "0.0.1",
+  "private": true,
+  "license": "MIT",
+  "scripts": {
+    "dev": "keystone dev",
+    "start": "keystone start",
+    "build": "keystone build",
+    "postinstall": "keystone postinstall"
+  },
+  "dependencies": {
+    "@keystone-6/core": "^5.0.0",
+    "@prisma/client": "^4.16.2"
+  },
+  "devDependencies": {
+    "prisma": "^4.16.2",
+    "typescript": "~5.0.0"
+  }
+}

examples/usecase-relationship-union/sandbox.config.json

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+{
+  "template": "node",
+  "container": {
+    "startScript": "keystone dev",
+    "node": "18"
+  }
+}