Extend Marketplace documents with external-name syntax

### What problem are you facing?

Currently, users cannot find the external-name annotation syntax before actually creating a resource. We can add a new section to the marketplace like Terraform registry's `import` and provide that information. This can be especially useful in import and migration scenarios. More context can be found [here](https://github.com/upbound/official-providers/pull/770#discussion_r980828137).

### How could Official Providers help solve your problem?
Automatically generate external-name annotation syntax for marketplace documentation.