Documentation missing mandatory metadata

### Describe the bug

When running `npx jsii-config`, `main` is a mandatory field for my package.json file.

### Expected Behavior

I would expect the documentation to state this as a required field.

### Current Behavior

Process fails and I have added `main` to my package.json file.

### Reproduction Steps

1. Create a package
2. Exclude the `main` field from the package.json file
3. Run `npx jsii-config`

### Possible Solution

Update the [author set-up](https://aws.github.io/jsii/user-guides/lib-author/quick-start/set-up/) guide to specify that `main` is a required field.

### Additional Information/Context

_No response_

### SDK version used

Not sure, failed before jsii was configured.

### Environment details (OS name and version, etc.)

MacOS 12.2 + Node.JS 14.18.3 + NPM 6.14.15

Start with the author set-up guide at https://aws.github.io/jsii/user-guides/lib-author/quick-start/set-up/ and review how package.json fields are described. Reproduce the issue with npx jsii-config and a package.json without main; done means the guide clearly identifies main as required.

Written by the indexing model from the issue text.