feat: support external JSON schema file references in type generation (#14749)

## Description

Enables the use of external JSON schema file references in type
generation by adding the `cwd` parameter to
`json-schema-to-typescript`'s `compile()` function.

## Changes

- Added `cwd: process.cwd()` parameter to `generateTypes()` compile call
- Enables `$ref` pointers to external `.json` schema files in
`typescriptSchema` field config
- Added test case with external schema file reference
- Documented external schema reference usage in generating-types.mdx

## Test Coverage

✅ New test: `resolves external schema file references`
- Created `test/types/schemas/custom-type.json` as external schema
- Added field with `typescriptSchema` referencing the external file
- Verified generated types include the external type correctly

## Example Usage

```typescript
// payload.config.ts
{
  typescript: {
    schema: [
      ({ jsonSchema }) => {
        jsonSchema.definitions.MyType = {
          $ref: './schemas/my-type.json'
        }
        return jsonSchema
      },
    ]
  }
}
```

External references are resolved relative to `process.cwd()`.

## Related Issue

This was identified during investigation of JSON Schema parser
capabilities. The `cwd` parameter is a standard
`json-schema-to-typescript` option that was not being passed.