api-gateway: allow adding documentation via L2

### Describe the feature

Right now, the only way to add documentation to resources is using the L1s (creating `CfnDocumentationParts`).

Request is to add a nice L2 API for this.

### Use Case

The use case mentioned by the customer is to start an API model in Swagger, and then add resources to it using CDK (which cannot be documented.

### Proposed Solution

_No response_

### Other Information

_No response_

### Acknowledgements

- [ ] I may be able to implement this feature request
- [ ] This feature might incur a breaking change

### CDK version used

newest

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

N/A

Start by tracing how the API Gateway L1 CfnDocumentationParts resource is exposed and how the existing CDK L2 API resources handle Swagger-backed models. Define the L2 API's scope and confirm done means users can add documentation to resources created through CDK while retaining the existing L1 capability.

Written by the indexing model from the issue text.