Provider Registry Protocol Not Included in Registry API Page

### Terraform Version

```shell
N/A
```

### Affected Pages

https://developer.hashicorp.com/terraform/registry/api-docs

### What is the docs issue?

It would be nice to include documents of the internal [Provider Registry Protocol API](https://developer.hashicorp.com/terraform/internals/provider-registry-protocol) in the [Registry API](https://developer.hashicorp.com/terraform/registry/api-docs) page to allow easier document finding.

### Proposal

_No response_

### References

_No response_

Open the affected Registry API page at developer.hashicorp.com/terraform/registry/api-docs and compare it with the Provider Registry Protocol documentation under terraform/internals/provider-registry-protocol. Add a clear reference to the protocol documentation on the Registry API page, then verify that readers can find it from the published page.

Written by the indexing model from the issue text.