Flag to tell registry-docs command that the provider was dynamically bridged

[guineveresaenger]

When we want to generate _index.md for dynamically bridged providers, we would like to emit slightly different information to the landing page for such a provider in the Registry.

We need a flag for the registry-docs command arg that tells the bridge to emit index docs specific to dynamically bridged providers.

[guineveresaenger]

This is implemented slightly differently via #2717.