Add model card links on keras.io for all pre-trained models

[mattdangerw]

As a follow up to #657, it would be great to add model card information for all our pre-trained models to keras.io.

There will be two parts to this work...

1. Add model card information to our preset metadata in this repository. We should include a URL to the original model card for the model, or link to a general purpose README if none is available. E.g. for a GPT-2 preset, we could add

"metadata": {
    ...,
    "model_card": "https://github.com/openai/gpt-2/blob/master/model_card.md",
},

2. Add code to render the model card somewhere within our preset tables on keras.io. See Keras NLP Autogenerate presets table.  keras-io#1222 as a reference.

[Cyber-Machine]

@mattdangerw I would like to continue working on this issue.

[mattdangerw]

@Cyber-Machine sounds great! You def have the most context on this anyway! Thank you.

[mattdangerw]

Re-opening so we can track landing keras-team/keras-io#1263 on keras.io (this will need to land after we release the 0.5 version of the library).

[mattdangerw]

All done!