improve documentation for registry providers

[dlqqq]

Problem

Registry providers like SM Endpoints and HF Hub lack clear documentation on how the model IDs are determined. Right now, we direct users to a provider's upstream documentation, but that doesn't help the user determine what model ID to specify.

We need to explicitly state how the local model ID is determined for each of the registry providers in the following areas of documentation:

• User documentation on RTD
• %ai list

Proposed Solution

Below is the current table of providers being shown in the user documentation on RTD. I would like to see an additional column in this table that:

• For non-registry providers: shows a collapsible list of valid local model IDs
• For registry providers: provides guidance on how to specify the local model ID. e.g. for SM Endpoints, "the local model ID is the endpoint name".

And similarly so for %ai list.

[JasonWeill]

Because the list of models is dependent on LangChain, I think it would be best to keep the exact list of models dependent on a call to langchain made in the course of handling an %ai list command.

I can update the documentation and the %ai list handler to refer to documentation for HuggingFace and SageMaker endpoint providers.

[JasonWeill]

Fixed by #254.